Abstract: Test of parabolic mirror is often completed by measuring the optical path length. Though it is highly accurate, it can only detect the smoothness instead of light convergence. Based on the basic principals, this paper describes the nature of convergence for parabolic mirror. It points out that convergence accuracy depends only on the angle between the direction vectors of surface elements and the incident ray instead of optical path length. Inthe end, methods for testing parabolic mirrors by collimated flood light sourceare provided.