As a new class of pollutants, microplastics have a large specific surface area and are easy to adsorb other pollutants. They are widely detected in soil and may affect the environmental behavior of phosphorus in soil. Phosphorus(P) plays an extremely important role in soil quality and nutrient cycling, and the loss of P from soil is also a major factor causing eutrophication of surrounding water. At present, there is no unified understanding of the effects of microplastics on soil P adsorption characteristics, and differences due to different content and particle size of microplastics is not clear. The mechanism of P adsorption in the microplastic-soil system formed after microplastics entering soil needs further study. In order to explore the effect and mechanism of microplastics on soil P adsorption, polystyrene microplastics (PS-MPs) were selected as microplastics samples, and soil from a farmland in Liaoning Province was sampled as the test soil. The P adsorption kinetics and adsorption isotherms of soil containing 0.1-10% microplastics were measured and analyzed. The effects of three different particle sizes of microplastics on soil P adsorption were investigated. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) were used to characterize the crystal compositions and functional groups of the microplastic-soil system before and after adsorption, to explain the effect mechanism of microplastics on soil P adsorption. The results showed that the adsorption process of P in the microplastic-soil system could be divided into three stages. In the first stage of the adsorption process, the rate of liquid film diffusion stage was significantly increased (p<0.05), and consequently increased the adsorption rate of P in the microplastic-soil system. Compared with the soil without PS-MPs (qe=6.456mg/g), the concentrations of 0.1%, 0.5% and 1% PS-MPs significantly reduced the P adsorption capacity of the microplastic-soil system (p<0.05). However, the P adsorption capacity of microplastic-soil system increased significantly with the concentrations of 5% and 10% PS-MPs (p<0.05). In addition, the adsorption experiments using PS-MPs with size of 48, 150, and 250 μm showed that the microplastic-soil system containing 48 μm PS-MPs had the highest adsorption capacity for P, followed by 150μm PS-MPs and 250μm PS-MPs. The XRD and FT-IR patterns of the microplastic-soil system before and after adsorption were compared to reveal the mechanism of the influence of microplastics on soil P adsorption. Minerals containing Si, Al and Ca in the soil were the adsorption sites of P and microplastics. Microplastics reduced the adsorption of P in the microplastic-soil system by competing with phosphorus for adsorption sites. Since microplastics can also directly adsorb P, the adsorption capacity of microplastic-soil system would increase when PS-MPs were more than 5%. In addition, the decrease of particle size of microplastics in soil would reduce the inhibitory effect of low content PS-MPs on soil P adsorption, and enhance the promoting effect of high content PS-MPs on soil P adsorption. This was due to the increase in the total specific surface area of the microplastic-soil system when the particle size of microplastics decreased. Therefore, microplastics pollution in soil can significantly change soil P adsorption characteristics, which is closely related to the content and particle size of microplastics.