For a pulse under the same stretch ratio there are two cases: one is the incident angle being less than Littrow angle and the other is the incident angle being bigger than Littrow angle. Investigation shows that the needed grating width is less in the former case than that in the latter case, and with the incident angle being one optimum value the needed grating width will reach the maximum in first case. Under the same condition the perpendicular distance between the gratings is also much less in first case than that in second case.