Beam expander is an essential part of ultrahigh peak power laser system. The reflective beam expander can solve the problem of dispersion, chromatic aberration, and pulse front distortion, which can be produced by transmission beam expander. The reflective beam expansion and focusing process based on ray tracing method and Huygens-Fresnel principle is analyzed. The influence of the off-axis parabolic mirror misalignment on the spatio-temporal characteristics of the laser pulse are calculated, when the parameters of the reflective beam expander system are different, and the error tolerance range of Strehl ratio and peak intensity is given.