【Objective】The mechanism of the inconsistent effects of straw application on crop yield is still unclear, and the main reason may be related to the insufficient understanding of the effects of straw application on soil microbial community composition and its abundance changes on crop yield.【Method】A pot experiment to study the growth of wheat was carried out using red and yellow-cinnamon soils. Three levels of straw application were set for each soil, namely 0, 10 and 30 g·kg^-1 soil(S₀, S₁₀, S₃₀). Based on bacterial-fungal co-occurrence networks, the Path analysis model was used to evaluate the contribution of microbial ecological clusters, enzyme activities and chemical properties to wheat yield. 【Result】The results showed that although the content of available nutrients, soluble organic carbon, microbial biomass carbon and enzyme activities(amylase, invertase, polyphenol oxidase, urease, acid phosphatase, dehydrogenase) increased significantly in both soils, the wheat yield increased with the increase in straw dosage in the red soil but decreased in yellow-cinnamon soil. Compared with S₀, S₁₀ and S₃₀treatments increased wheat grain yield and above-ground biomass by 33%-44% and by 73%-85% in the red soil; and decreased wheat grain yield and above-ground biomass by 22%-25% and by 55% in the yellow-cinnamon soil, respectively. The abundance of two key ecological clusters within the bacterial-fungal co-occurrence network, enzyme activities and soil chemical properties had positive effects on wheat yield in red soil while the abundance of two key ecological clusters had a larger positive effect on wheat yield in yellow-cinnamon soil. The straw application significantly increased the abundance of Aspergillus, a key microorganism positively correlated with wheat yield in red soil, while significantly decreased the abundance of Bacillus, Burkholderia, and Basidiobolus, which were positively correlated with wheat yield in yellow-cinnamon soil.【Conclusion】The combined effects of straw application, an increase in key microbial abundance, enzyme activities, and improvement of soil chemical properties was responsible for improving wheat yield in the red soil. In the yellow-cinnamon soil, the decrease in wheat yield was mainly related to lower key microbial abundance, whose effect was superior to the potential positive effects of improved soil chemical properties and enzyme activities. These results suggest that the change in the abundance of key microorganisms has an important influence on the variation of crop yield after straw returning.