It needs a down-collimator to convert big beams of a large laser facility into smaller beams in diameter to fit measurement instruments in diagnostic system. Because ultra-short pulses have wide spectrum, dispersion will be important to measurement. This paper provides some advice about diagnostic system designed by analyzing the down-collimator’s characteristic and group velocity dispersion of the sampling mirror. The result shows that the edged sampling mirror and the edged vacuum window can be treated as a pair of prisms, which generates negative group velocity dispersion. And the group velocity dispersion increases with the edged angle and the optical path. When input pulse width is above 50 fs, the output one will not change. So we can obtain the work range of this diagnostic system.