A measuring path for grating replication and mosaic was designed based on interferometric method. In consideration of the difficulty of measuring incident light angles in the optical path for grating replication and mosaic, the influence of the angle error of the incident light on the grating replication and mosaic was analyzed. A mathematical model of mosaic grating errors was established, and the tolerance of the mosaic grating errors was given. According the requirement of incident light angle in the optical path for grating replication and mosaic, an angle adjustment structure of the interferometer was designed. On the basis of error mathematical model and the mosaic path, the relation between the incident angle error and the mosaic grating errors was analyzed for a 5 000 mm×500 mm large echelle grating. The results show that when the incident angle error is 1°, and the differences of calculation value and actual value for the Δθx, Δθy, Δz( mosaic grating errors around the x,y,z axes) are 0.002 1 μrad，0.003 3 μrad，0.348 2 nm, respectively, the wavefront error is 2.590 1 nm. Accroding to the calculation and analysis, it shows that the angle adjustment structure of the interferometer needs a dividing plate of 0.1° as the min measuring range , which can meet the requirement of the grating mosaic.