An improved Linnik interferometer for phase-shifting digital holographic microscopy in reflection configuration is demonstrated. Some phase-shifting configurations which have been used are analyzed and compared. Higher phase-shifting precision can be achieved in this improved Linnik interferometer, so the microscopic imaging quality can be improved. In experiment, to prove its feasibility, the 3D microscopic imaging of the USAF-1951 target surface structure has been realized, the amplitude and phase distributions of object wave are reconstructed.