The invention provides a method for improving soil through rice straw returning. The method comprises the steps that firstly, rice straw is pretreated, the pretreated rice straw is stacked into a long trapezoid, and a biological agent is adopted for fermenting the rice straw to obtain straw a composted substance. The adopted biological preparation has the powerful composting effect on straw, in the fermentation process, a large number of functional bacteria are bred, and various special-effect metabolites are generated, and therefore growth and development of crops are stimulated; crop disease resistance, drought resistance and cold resistance are improved, after functional bacteria enter soil, nitrogen fixation, phosphorus dissolving, potassium dissolving can be achieved, the soil structure can be improved, and the chemical fertilizer utilization rate can be increased.