Effect of long-term fertilization on components and changes of phosphorus in fluvo-aquic soil was studied in Tianjin during 1979-1993. Results showed that inorganic phosphorus was predominant, in which Ca-P occupied the largest percentage, O-P was second only to Ca-P and only small part was the Al-P and Fe-P. Long-term fertilization could significantly influence both the content of soil available P and the range of its changes. The effects of chemical phosphorus were more obvious than those of organic sources. The increasing and decreasing of soil available P depended heavily on whether chemical phosphorus was applied or not, soil available p increased or decreased at the rate of 1 mg/kg when 45kg P/ha was added or removed. In addition, our study also suggested that the changes of soil available P differed with the periods of experiment at time. Generally speaking, soil available P changed largely in the first two years, and the changes became smaller in the later 6-8 years, finally it remained in a normal value with quite small changes. Soil phosphorus could be balanced in 15 years when 18 kg/ha. year P₂O₅ was lied for per 100 kg of grain yield under the wheat-corn cropping system with the grain yield of 11400kg/ha in total.