In the design of cubic phase plate testing system by taking Computer Generated Hologram(CGH) as a null lens,two very important principles are the wavefront aberration of cubic phase in propagation and the separation of diffraction orders.The cubic phase introduced higher order aberration in propagates was presented,and an expression for the aberration of cubic phase in propagating was deduced in detail.From the expression,it was shown that the high order aberration increases as the propagation distance l increase.The theoretical foundation for the diffraction order separation of testing wavefront diffracted by the CGH was discussed.To prove the theories,a cubic phase plate testing system was designed for an example.By introducing the 4th and the 5th order items,the wavefront aberration reduced from 0.7 λ to 0.007 λ.The results coincide with the theoretical formula well.