Abstract: Aiming at the difference of moving electromagnetic（EM） field between the EM rail launch and the traditional, this paper deduced the control equations and boundary conditions of EM rail launch moving EM field systematically, and established its numerical calculation model. The method that use fluent user defined scalar（UDS） secondary development to calculate the model was put forward innovatively. The impact of velocity skin effect（VSE） on internal magnetic field of EM rail launch projectile was simulated. Simulation results show that contrast to the case without consideration of VSE the maximum magnetic flux density（MFD） in the central line of projectile decreases from 5.6 T to 5.2 T, and the peak moment of MFD was brought forward from end moment to start moment of the advection section of discharge current. The method in this paper can provide basis for the modeling of in-bore multi-physical coupling of EM rail launch, and the research results can provide design input to the construction of simulation platform of launching magnetic field of EM launch projectile.