In order to test optical figure of aspheric off-axis segments of extremely large telescope, we present a new optical testing method. Take advantages of transmission and diffraction nulls, the aspheric departure is accommodated by both aplanatic lens and computer generated holograms. For plenty of large off-axis aspherical mirrors, only one optical path needs to be built. Different segments will be measured by replacing computer generated hologram (CGH). Design results show that, for different off-axis magnitudes as 2.5 m, 8 m and 14.5 m of a primary mirror with 60 m vertex curvature radius, conic constant K=-1.000954, parent mirror diameter Φ30 m, segment diameter Φ1.5 m, all can achieve self-collimation null tests in one light path structure.