The slip step Fourier transform method is adopted to study the coupled-wave equation of noncollinear autocorrelator. The influence of spatial intensity distribution and pulse chirp of the fundamental beams on measurement of pulse duration of second-order autocorrelation signal are simulated. Two fundamental beams and only one fundamental beam modulated by spatial intensity distribution are respectively discussed. It is shown that the irregular spatial intensity distribution modulation of the second harmonic beams increases with the fundamental beam modulation and decreases with modulated intensity increase of the fundamental beam. The outside envelope of the autocorrelation with irregular spatial intensity distribution is the same as that of the autocorrelation without modulation. The pulse chirp only leads to efficiency decrease of autocorrelation. The method of pulse duration derived from outside envelope of autocorrelation is presented. The analyzing results are of reference value for the exact measurement of pulse duration.