[Aim] To study the impact of returning residues of genetically modified crops to soil on fertility characters.[Method] Several Bt cotton varieties with different resistance to insect and non-transgenic conventional cotton (Simian no. 3) were used as research materials in this study. All the plants were mechanically smashed and returned in site after one or two growth cycles. 40 days later, Bt protein content and the nutrient contents in soil were detected and analyzed to reveal the effects of Bt cotton straw returning on the soil fertility characters.[Result] The results showed that straw returning would not distinctly result in the increasing of Bt protein contents in soil for all cotton species. And moreover, there was no significant difference between Bt transgenic cottons and non-transgenic cotton in content variation of soil Bt protein. The amount of organic matter, available phosphorus, alkali-hydrolyzed nitrogen, available potassium, total nitrogen, total phosphorus and total potassium can be significantly increased by cotton straw mulching, and the soil pH value can be improved too. No significant differences in change scope were found between Bt cotton and non-transgenic cotton. The above results are further confirmed by the grey relation analysis, and the effect of straw returning on soil fertility was not related to the insect resistance of Bt cotton.[Conclusion] The improvement of soil fertility by straw mulching had nothing to do with the insect resistance of Bt cotton. "Bt gene" does not become a restrictive factor for Bt cotton straw returning to improve soil fertility. The soil fertility quality was not adversely affected by the straw returning. Instead, it can increase soil nutrient content and effectively improve soil fertility. In sum, straw returning is so simple and harmless that could be used as a potential optimal choice for transgenic plants disposal.