【Objective】Straw biochar is a kind of carbon-rich material prepared through pyrolysis under high temperature in anoxic condition. Its application may directly or indirectly affect availability of soil phosphorus in the soil through altering soil physicochemical properties or soil environment during the crop-growing season. Freezing and thawing alternation is a common climate phenomenon in Northeast China. Frequent occurrence of such a phenomenon in the spring causes changes in soil properties like soil structure, thus leading to drastic variation of soil available phosphorus content. However, so far little has been reported about the effect and mechanism of biochar affecting availability of soil phosphorus during the freeze-thaw season. In this study, an indoor simulation experiment was conducted to explore rule and mechanism of biochar affecting availability of soil phosphorus in the brown earth of Northeast China during the freeze-thaw cycle and its impact on soil nutrient suppling capacity during the early crop growing season in the region. The findings in this study may have some significance to guiding the theoretic study on use of biochar practice of and theoretic study of use of biochar in Northeast China. 【Method】Soil samples were collected from a maize field in the Comprehensive Field Experiment Base of the College of Water Conservancy, Shenyang Agricultural University, after the harvest in 2015, and then air-dried and sifted for future use. Biochar was prepared out of maize stalk and ground to pass a 1mm sieve. Then the biochar was blended with air-dried soil samples at a rate of 0% (0﹕100), 2%, 4% and 6%, separately. The mixtures were them packed, separately, into 20 cm×20 cm×15 cm plexiglass boxes, 2.5 g each, constituting four treatments and three replicates each. All the samples in the boxes were incubated under room temperature for 60 days. During the incubation, the samples were kept wet with soil moisture content being 50﹪ of the soil water holding capacity by adding distilled water weekly. After the incubation, the soil samples were subjected to 30 cycles of simulated freezing-thawing with temperature varying between -10 ℃ and 7℃. A set amount of the soil sample in each box was retrieved after 0, 1, 3, 5, 10, 20 and 30 cycles of freezing-thawing for determination of soil properties, including content of readily available phosphorus, pH, organic matter, electric conductivity and activity of phosphatase.【Result】 (1) The effect of the alternation of freezing and thawing was extremely significant on content of soil available phosphorus during all the phases of the treatment (0 ~ 5 cycles, 5~30 cycles and 0 ~ 30 cycles), relative to number of freeze-thaw cycles, biochar application rate and their interactions, except for the first five cycles. (2) After the incubation, the content of available phosphorus was increased by 24.0%, 25.7% and 42.5% in Treatments 2%, 4% and 6% as compared with CK separately. Obviously the effect increased with rising biochar application rate. The effect peaked during the 5th cycle, however, the increment declined with rising biochar application rate. Around the 20th cycle, the contents of available phosphorus in all the treatments dropped down to relative valleys, available phosphorus and even lower that that in the treatments applied with biochar and incubated under room temperature, which indicates that biochar increased the content of available phosphous under room temperature, while the freeze-thaw cycles helped boichar fix soil phosphorus, as compared with CK, thus reducing the risk of phosphorus loss with melting snow. (3) The analysis of responses of biochemical properties of the soil, such as soil pH, EC, organic matter and phosphatase activity, to freezing and thawing cycles and relationship of soil available phosphorus with phases of the freeze-thaw cycles reveals that the content of organic matter varied sharply with the freeze-thaw cycle going on and was closely related to soil available phosphorus.【Conclusion】Through enhancing the stability of soil aggregates, biochar helps soil organic matter hold soil phosphorus by reducing its release. During the freeze-thaw cycles, biochar helps fix soil P and reduce P loss with thawing snow.