Mosaic grating is one of the most important methods for fabricating large-size plane diffraction gratings, and mosaic error is an important index to evaluate whether the mosaic grating can be used. Therefore, the real-time quantitative measurement of mosaic error can realize automatic closed-loop adjustment and improve the accuracy of mosaic grating fabrication. The mathematical model of the diffraction wavefront and the mosaic error of the grating is established and the mosaic error is analyzed. The diffraction wavefront of the mosaic grating at the zero-order and the negative first-order is extracted quantitatively by using a ZYGO interferometer. The numerical solution of five dimensional error is calculated by analyzing the mosaic error wavefront, and its accuracy is verified by the negative second-order diffraction wavefront. The experimental results show that the mosaic error of the gratings at the zero-order, the negative first-order and the negative second-order is consistent with each other, which provides theoretical guidance for measuring mosaic error by wavefront and realizing automatic closed-loop adjustment.