The model of spectral clipping due to the tiled-grating gap of parallel grating pair compressor is established for the circular input beam, and the signal-to-noise ratio caused by the spectral clipping is simulated. Compared with sech2 pulse, the impact of tiled-grating gap on the signal-to-noise ratio of Gaussian pulse is less serious. With the tiled grating gap width decreasing and the beam diameter increasing, the signal-to-noise ratio of the compressed pulse can be improved. For a certain tiled-grating compressor and beam diameter, the highest signal-to-noise ratio is available with the central wavelength of the pulse equal to the central wavelength of the spectral clipping (the central wavelength of the wave band clipped by the tiled-grating gap).