Microplastics are abundant in agricultural soils and have significant impacts on rainfall infiltration and soil water-retaining capacity. To explore the effect of microplastics on agricultural soil permeability by simulating the rainfall irrigation process, a one-dimensional vertical soil column rainfall infiltration test device was used to study the unsaturated infiltration characteristics of loess soil imbued with microplastics under rainfall conditions. The following conclusions could be obtained: the microplastic content (q), the microplastic particle size (p), and the soil density (γ) have effects on rainfall infiltration; the soil water-retaining capacity would be weakened owing to the existence of microplastics; and intermittent rainfall is preferred in agricultural irrigation. Finally, the permeability coefficient (k) and average flow rate (V) of the unsaturated soil are deduced together, and the relationship between the permeability coefficient (k) and the matrix suction (ψ) of the unsaturated loess soil containing microplastics is calculated by an example, proving good consistency between the experimental results and theoretical calculations. Microplastics represent negative effects on rainfall infiltration and soil water retention, so it is recommended to dispose of them.