Microplastics are widely present in soil， and the problem of microplastic pollution in soil has been considered during recent years. Three typical farmland soils including the paddy soil， the red soil and the fluvo-aquic soil were used to conduct indoor incubation experiments for 70 days to study the effects of microplastic pollution on the mineralization of organic carbon in soil under rice straw returning. 4 treatments including the control （CK）， addition of microplastics （polyethylene， PE）， addition of rice straw （RS）， addition of microplastics and rice straw （RS-PE） were set for each type of soil. The physical and chemical properties， the emission of CO₂， the content of soluble organic carbon （SOC） and microbial biomass carbon （MBC） in each soil treated were measured. The results showed that the cumulative emission of CO₂ from three types of soils within 70 days under the PE and RS-PE treatment was in the order of fluvo-aquic soil>paddy soil>red soil. Compared with CK， the cumulative emission of CO₂ from three types of soils significantly increased after RS treatment， and the cumulative emission of CO₂ from the red soil and fluvo-aquic soil significantly increased after PE treatment. Adding PE and RS increased the content of SOC and MBC in three types of soils， promoting the mineralization of organic carbon in soil. The E₂/E₃ value in three types of soils was in the order of paddy soil>red soil>fluvo-aquic soil. The addition of PE and RS increased the average relative molecular weight， aromaticity， and hydrophobicity of SOC in soil. The mineralization of organic carbon in three types of soils was significantly and negatively correlated with the average relative molecular weight of SOC （E2/E3）， and the mineralization of organic carbon in paddy and fluvo-aquic soil was significantly and positively correlated with the aromaticity （SUVA254） and hydrophobicity （SUVA260） of SOC. It is indicated that t the addition of microplastics and rice straw significantly affect the mineralization of organic carbon in soil.