By simplifying the rail as a Bernoulli-Euler beam fixed on the elastic support under moving loads and obtaining the equivalent stiffness of the elastic support through the finite element model, the critical velocity of the launcher was derived. Dynamic contact force between the armature and the rail and the distribution of stresses and strains were obtained from electromagnetic-structural-motion multi-physics field coupling dynamic model by using hybrid finite-element/boundary-element method. The measuring data of fiber Bragg grating strain sensors on the back of rails were used to verify the dynamic response and analyze armature stability. For the typical 30 mm × 30 mm rectangular caliber launcher, analysis and test results show that the electromagnetic extrusion force of the C-type armature on the rail reaches the peak value at the beginning of the flat-top and gradually decreases as time goes on; the stress waves caused by armature passing are easily resonant with the reflected stress waves in the rail in the high-speed stage, in which the stress exerted on the rail is the most severe and the stress concentration level is about 2.44 times that in the initial region; when the armature operates in the high-speed region, there will be a vibrating phenomenon which will lead the asymmetry of loads on the upper and lower rails. These results have guiding significance for analyzing dynamic response characteristics of internal ballistic process and design launcher.