Microplastics are emerging pollutants widely present in soil, and their effects on soil phosphorus (P) adsorption are still unclear. This paper examined the effects of 0.1% to 10% microplastics on soil P adsorption and the underlying mechanisms. The results showed that microplastics could significantly increase the rate of liquid film diffusion stage in the first stage of the adsorption process (p<0.05). Compared with the pure soil (q_e=6.456 mg/g), the soil P adsorption capacity was significantly reduced by microplastics with less than 1% concentration (p<0.05). However, the soil P adsorption capacity was significantly increased by microplastic with concentrations higher than 5% (p<0.05). Additionally, the smaller the particle size of microplastics, the greater the P adsorption capacity of soil with the same microplastic concentration. Microplastics could compete with P for adsorption sites, which reduced the adsorption of P in the microplastic-soil system. However, microplastics could also directly adsorb P, leading to an increase in the P adsorption capacity of microplastic-soil system when microplastic concentration were more than 5%. Therefore, microplastics pollution in soil could significantly change P adsorption characteristics, which closely related to the microplastics concentration and particle size.