Inductance gradient and resistance gradient are core parameters of electromagnetic rail launcher, affected by the diffusion process and the velocity skin effect, the current density distribution of the rail during the emission process changes with time and space, and the electrical parameters have dynamic characteristics. The equivalent frequency is introduced according to the current diffusion equation to simulate the change of skinning depth during the armature launching process, so as to calculate the armature motion process equivalently using the armature static simulation results. A parameter extraction method is proposed to solve the corresponding equivalent frequency by calculating the current skinning depth during the launching process through the microelement method, and the dynamic electrical parameters of the system are extracted by combining with the results of finite element simulation. The method is used in the whole system simulation, and the error between simulation and experiment is within 1%, which proves the effectiveness of the method.