For the measurement of single shot femtosecond laser pulse, to describe the influence of incident angle and crystal rotation on the output signal accurately, the third harmonic process is analyzed and deduced. In this article, the range of the angle of incident light is derived and calculated mathematically. At the same time, the relationship between the output autocorrelation signal and the crystal’s attitude is analyzed quantitatively. The results show that, to satisfy the phase matching condition, the minimum value of the incident angle is 30.114°. The output direction of the autocorrelation signal is insensitive to the rotation of the crystal. However, the output intensity is sensitive to the intersection angle of two incident beams. The capacity of the crystal rotation is large, and the thinner the crystal is, the less sensitive it is. Moreover, the time resolution of the correlation signal is determined by the angle between the two incident beams in the crystal, the change of the angle outside the crystal does not change the time resolution. The study has reference value for the precise adjustment of femtosecond laser pulse measurements.