In the high-power ultrafast laser system, interaction between the high-power laser and target substance would produce plasma,in order to know the evolvement of the plasma,the delay time between probe laser and main laser should be controlled accurately, in which the precision of delay need to be several picoseconds even several femtoseconds. Therefore an instrument, which was based on the ultrafast pulses crosscorrelation theory, by utilizing the sum frequency generation effect, CCD to collect the spatial intensity distribution of sum frequency beam, was designed. By measuring the shift of curve peak, the delay between fundamental beams can be calculated, the fundamental beams can be accurately delayed by controlling the optical layout. The conformation of delay instrument was described, the principle of the ultrafast pulses crosscorrelation measurement was discussed, the relationship between the delay time and the shift of the curve peak was analyzed theoretically. The influence on the measure precision caused by the angle of fundamental beams was discussed numerically, to reach a much more accurate delay time, we can change the nonlinear crystal according to the beam angle.