Optical freeform surface acts an important role in modern optical system. Computer generated hologram which can generate arbitrary wavefronts shows great advantages in the null test of optical freeform surfaces. According to an off-axis aspheric surface, the surface shape deviation can be gotten by testing the initial aspheric surface with Offner compensator. The off-axis aspheric surface lacks of symmetry can be regarded as freeform surface by rotation and translation in the optical path, and a CGH null compensator is designed on flat substrate to achieve null test of the freeform surface. With calculation and analysis of the main errors in the CGH null test, the measurement uncertainty of freeform surface test can reach λ/10[peak- valley(PV) value] level. CGH compensator can be used in the high precision test of freeform surface.