Abstract: The phase of sample point should be calculated to design the computer generated hologram(CGH) for testing off-axis aspheric surface. The normal of the geometric center of off-axis aspheric surface was seen as the detection optical axis through shift and rotation. Then the phase distribution on off-axis aspheric surface was studied by ray tracing, and a calculation method of discrete phase was deduced, as well as the formulas for phase computation in three-dimensional coordinate system. The validity of this method is proven by calculating and comparing the phase distribution of a rotational symmetry aspheric surface. The results show that this method is correct, and the calculation accuracy can meet the requirement of CGH for off-axis aspheric surface test.