【Objective】 About 30%~ 65% of the total P in soil is present in the form of organic P, mainly like phospholipids, phytate, and nucleic acids, which are not readily available to plant root. However, organic P would release, through hydrolysis, phosphate ions which are available to plant root. Phosphatase is an important enzyme to hydrolyze organic P to phosphate. Furthermore, nutrients in organic manure are important to higher watermelon quality, but the mechanism of how watermelon root absorbs soil P is not clear. The objectives of this study were to (1) investigate forms and quantity of the phosphorus nutrient supplied by organic manure; (2) validate whether the organic acids secreted from watermelon root influence availability of the P in organic manure; and (3) demonstrate responses of the acid phosphatase secreted by watermelon root to organic manure application. 【Method】 The analogy method was adopted to determine contents of water soluble total P and inorganic P in organic manure, as well as impacts of the organic acids secreted from watermelon root on concentrations of soluble total phosphorus and inorganic phosphorus in organic manure. A sand culture experiment was conducted to explore responses of the acid phosphatase in activity to substitution of chemical fertilizer P with organic manure P. In addition, a field experiment was carried out to investigate effects of the replacement and application rate of organic manure on acid phosphatase activity in rhizosphere soil, content of available P in rhizosphere soil, and phosphorus nutrition, yield and quality of the watermelon. 【Result】 Results show the organic manure used in the experiment contained 6.9 g·kg^-1 soluble total phosphorus and 525.1 mg·kg^-1 soluble inorganic phosphorus. Of the soluble total phosphorus 7.6% was inorganic P and 92.4% organic P. Organic phosphorus could not be absorbed by root until it is hydrolyzed. Organic acids, either citric acid, malic acid or succinic acid, secreted from watermelon root increased soluble total P content, but did not influence the content of inorganic P in the organic manure. Citric acid, low or high, increased the content of soluble total P significantly or by 78.9% and 223.6%, respectively, while high concentration of malic acid and succinic acid did by 151.5% and 112.8%, respectively, in the organic manure. But high concentrations of citric acid and succinic acid did reversely. Substitution of chemical fertilizer with organic manure lowered the accumulation of dry matter in watermelon. Activity of the acid phosphatase in the rhizosphere of watermelon root was significantly enhanced when chemical fertilizer P was replaced by organic manure P. And root/shoot ratio of the plant was improved by the substitution. The field experiment demonstrates that the application of organic manure (6 t·hm^-2) to replace chemical fertilizer did not affect yield of the crop. The application of organic manure at 18 t·hm^-2, triple the replacement rate, significantly increased yield of the crop, and the application of organic manure at 6 t·hm^-2 or 12 t·hm^-2, double of the former, both improved the activity of acid phosphatase in the rhizosphere. However the application at 18 t·hm^-2 did not affect much the activity, nevertheless it did increase the content of available P in the rhizosphere soil at the mature stage of the crop. 【Conclusion】 The organic manure does contain a small amount of soluble inorganic P, available directly to root for absorption, but most of the soluble P is organic P, hard to be absorbed by root before is hydrolyzed. The organic acids secreted from watermelon root improve soluble total P in content, but not soluble inorganic P. However, the soluble organic P increased by the organic acids secreted by watermelon root is still not available for absorption before it is hydrolyzed. The substitution of chemical fertilizer P with organic manure P enhances the activity of acid phosphatase secreted watermelon root and the capability of the crop of utilizing organic P. Acid phosphatase is higher in activity in the treatment applied with organic manure than in the treatment applied with chemical fertilizer, equivalent to the former in P application rate. However, excessive application of organic manure does not further enhance its effect on activity of acid phosphatase. Application of organic manure at a higher rate increases the content of available P in rhizosphere soil and hence P absorption and watermelon yield.