An interference method to remove all mosaic grating errors using a phase-difference reference window without compensation for coupled mosaic errors or analyzing the far-field diffraction intensity patterns is proposed. The reference window is achieved by adding a small-aperture prism; therefore, the incident light on the mosaic gratings contains two different angles of incidence and optical paths, eliminating the uncertainty of longitudinal mosaic error in the interferometer. The mathematical model of the method is established and optical arrangement is presented. Interference fringes with the reference window and alignment process are simulated and the accuracy of mosaic errors are analyzed. The experimental and simulation results show that our method can eliminate longitudinal mosaic error and all other mosaic grating errors. The accuracy of angle mosaic errors can be less than 1 μrad and position mosaic errors can be to the nanometer level. Compared with the far-field diffraction intensity patterns method, our method reduces the complexity of the optical structure and ensures measurement accuracy.