In order to study the law of salt transport in loessial soil with diffferent particle sizes and contents of spherical microplastics under different rainfall conditions, a vertical infiltration experiment was carried out by using the indoor one-dimensional soil column model, and the scatter diagram of the infiltration conductivity of soil columns with different contents and particle sizes of microplastics under different rainfall conditions was obtained. The results showed that a low content of microplastics in loessial soil significantly promoted the infiltration of NaCl solution, while a high content of microplastics blocked the infiltration of NaCl solution, and the rainfall conditions changed the critical content of microplastics. In loessial soil, a slightly smaller particle size of microplastics significantly promoted the infiltration of NaCl solution, and a slightly larger particle size of microplastics blocked the infiltration of NaCl solution. The microplastics with lower content and smaller particle size are more sensitive to the rainfall response.