A method for the vertical downward separation experiment of small satellites is adopted to solve the problem of limited space for the separation of a small satellite from a large spacecraft.Based on the coordinates of high-speed photographic target obtained from the vertical downward separation experiment of a small satellite on a large spacecraft，a calculation model for the separation parameters of the small satellite under zero gravity is derived and established，and the separation velocity，separation angular velocity，and attitude angle of the small satellite are obtained.Based on the Monte Carlo method，a simulation analysis for the separation kinematics of the small satellite is carried out.The simulation results obtained are consistent with the separation experiment data，which verifying the correctness of the calculation model and the effectiveness of the experiment method.This separation experiment method can be used for similar small satellites.