A method based on basic phase measuring deflectometry (PMD) is proposed for testing the aspherical mirror,which uses two reference screens in different distances from the tested mirror. The intensity-modulated patterns are generated on the screens and the images reflected via the tested mirror are recorded by a camera. The original ray of every pixel and its corresponding deflected ray can be constructed using the recorded patterns and phase-shifting technique. Their intersection points and the surface normal are obtained. Then the mirror surface is reconstructed by numerical integration. The procedure for this method is presented,and the computer simulation and experimental results are shown.