The asymmetry of ultrashort light pulse may be detected by third order intensity correlation. As compared with the results of SHG method, more information may be obtained about the shape of the pulse. According to our measurement, the shape of the pulse of passively mode-locked Nd: YAG Laser can be considered approximately as Gaussian-exponential. The correlation width obtained by third order intensity correlation was 22.3 ps+15.7 ps, its corresponding pulse width was about 27ps. The leading edge of the pulse was steeper than the trailing edge.