Abstract: The method of combining reference objective and numerical fitting is established to solve the phase distortion of off-axis digital holography. Because of the optical curvature difference caused by the objective， the pre-amplified digital holographic microscopy system often produces additional phase factors. By introducing the same reference objective in the reference beam path， the phase distortion due to curvature problems is initially eliminated. With the addition of the reference objective， phase distortion mainly comes from three aspects: the numerical simulation of plane reference light is used in diffraction calculation， and a phase distortion is generated with the off-axis structure having a spatial angle; the actually recorded hologram causes a secondary phase distortion because the object wave passes through the pre-amplified objective; the other optical components in the optical path form aberrations， failing to get the correct 3D shape of the microstructure. The entire phase surface is fitted by a polynomial， and the difference between the phase surface and the fitted surface is used to extract the true phase distribution of the sample. With only one hologram， multiple phase distortions are automatically compensated by a polynomial fit. The USAF resolution plate being used as the microstructure pattern for measurement，the accuracy and effectiveness of the method are verified.

Key words: off-axis, digital holography, microscopic, reference objective, polynomial fitting, distortion compensation