Abstract: The non-linear 2ndharmonic conversion effect of the KDP crystal is researched and its effect to measurement accuracy of the laser pulse width. Experimental results show that: in condition of small signal and Gaussian beams, the 1stharmonic laser pulse width is times of that of the 2ndharmonic one with type Ⅱ KDP crystal whose thickness is 1～3cm; Due to the nonGaussian beams, the halfwidth of the 1stharmonic pulse equals to the quarter width of the 2ndharmonic pulse. With higher power density and energy conversion efficiency, the rate between the 1stharmonic pulse width and the 2ndharmonic one vary seriously．