Based on the requirement of the clean narrower second harmonic probe pulse, the Slip Step Fourier method is used to analyze the frequency doubling process in the third-order noncollinear correlator.The evolution characteristics of temporal profile are simulated for the Gaussian pulses and type I phase-matched of KDP crystal.The range of fundamental incident laser energy can be acquired with the second harmonic pulse shape remaining the Gaussian distribution.The pulse width of the third-order autocorrelation differences with the different ratios of full width of half maximum of fundament to second harmonic pulse at the different conversion efficiency of frequency doubling.This leads to the variable convolution factor of pulse width concerned with the pulse shape.The analyzing results are of reference value for the parameter design and error evaluating of the third-order autocorrelator.