Slope soil erosion is common in hilly regions and reservoir bank slopes, as the primary driving force is brought by the thin overland water flow caused by rainfall or wave run-up, which results in the shear force and subsequently the incipient motion of soil. In order to explore the incipient motion conditions of slope soil under waterflow scouring, the incipient motion test and theoretical research of slope soil were carried out via self-developed testing device. The incipient motion phenomenon of cohesionless soil particles was observed by particle staining and high magnification electron microscopy, and the relationship between incipient motion mode of cohesionless bank slope soil and flow velocity was determined. This study also explored the relationships among differing dry densities, clay contents, and slopes with the critical incipient motion velocity on a clay slope, the results showed that clay content, dry density, and soil gradient had a great impact on the incipient velocity. Compared with the dry density and gradient, clay content had a clearer impact on incipient velocity. The incipient motion velocity equation of cohesionless bank slope soil was verified, and the rolling incipient motion velocity equation was reliable. The incipient motion mode of clay was used to establish the mechanical balance equation of the clay slope. Then, a semi-empirical incipient motion velocity equation of the clay slope was deduced, of which the relevant parameters were solved using the test results. The proposed incipient motion velocity equation showed good fit with the test results, and the verification results demonstrate that the equation is reliable.