Abstract: Effects of the tilt of a phase mask on wavefront coding systems are analyzed and the generalized pupil function is derived by a coordinate transformation in this paper. The approximate expression shows that the tilt of the phase mask brings a magnification effect on the cubic phase mask coefficient. The effect is independent on the sign of the angle, however, it will get larger with the absolute value of the angle increases. Moreover, the tilt of the phase mask extends both the sides of the Point Spread Function(PSF) envelope and causes a decrease of Modulation Transfer Function(MTF). When the tilt is in the meridian, the magnification effect of cubic phase mask coefficient in the meridian plane is larger than that in sagittal direction, which results in larger extension of PSF and lower MTF in the sagittal direction. All above conclusions are proved by the simulation with MATLAB and optical software.