The influencing factors of laterite starting on slopes under the effects of rainfall were revealed. The starting conditions of laterite starting on slopes under the effects of rainfall were established, in order to provide a reference for further study on slope erosion of laterite in Yunnan. According to the characteristics of laterite and the erosion processes and phenomena on the slope surface, the method of combining the model test and the mechanical model was adopted. The results indicated that: (1) The splash erosion and crust were the two main phenomena in the processes of erosion of laterite on slopes. (2) The cohesive force of laterite could block the starting of laterite particles on slopes. The greater the shear strength, the greater the critical shear stress of the laterite particles. Rainfall and pore water pressure could promote the starting of laterite particles on slopes. The greater the raindrop density and the final velocity , the greater the impact of rainfall. When the particle size d was larger and the ratio of raindrop diameter to water depth decreased, the effect of rainfall was smaller. (3) The formulas of the critical starting shear stress of laterite particles in the connection and loose state were established. The critical starting shear stress of laterite particles on slopes was closely related to the particle size, shear strength, rainfall characteristics, and runoff characteristics. Under the effects of rainfall, a small flow shear stress could make the laterite particles under loose state start.