To analyze the assembly error of the interferometer plate in the Mirau interference-microscope objective, we create a characterization model of interference light intensity of a tilted reference plate. By dividing the test beam and reference beam in the pupil plane, the integral expression of the interference light intensity is obtained when the reference plate is tilted. The influence of the tilted reference plate on the envelope and contrast of interference fringes is quantitatively analyzed by numerical simulation. Then, the tilt error tolerance of the interferometer plate is determined by the results of three-dimensional morphology reconstruction using an eight-step phase-shifting algorithm for broadband light. An experimental verification structure consisting of a Mirau interference-microscope 50× objective is fabricated and a standard step is experimentally measured to verify the accuracy of the simulation model. Results show that the tilt angle of the interferometer plate in a Mirau interference-microscope 50× objective should be constrained to be less than 1.5°.