This study is aimed to investigate the effect of rice straw returning depth on soil phosphorus bioavailability in paddy field，and explore the optimum straw returning depth for yellow-brown paddy soil，and provide scientific basis for improving straw utilization rate and soil physicochemical properties. Through the indoor culture experiment，the rice straw was returned to the field at 0，10，20 and 30 cm soil layer，and two conditions were set up:adding decomposing agent and not adding decomposing agent. The results showed that the decomposing rate was basically the highest in each sampling period in the treatment of returning to the field to 10 cm soil layer and adding decomposing agent，and the rate of phosphorus release from straw was also the highest. The bioavailability of phosphorus was improved after the straw was returned，and the content of stable inorganic phosphorus and active inorganic phosphorus in each soil layer was mainly improved. After 12 weeks of straw returning to the field，the 10 cm soil layer treatment showed the largest increase in phosphorus bioavailability，compared with no straw control of 10 cm soil layer，the moderately stable inorganic phosphorus content in the S10 and S10+F treatments were increased by 44.04 and 50.48 mg/kg，respectively. The 30 cm return to the field had the smallest improvement. Therefore，the best effect is to apply decomposing agent at a depth of 10 cm when straw is returned to the field.