Laser ultrasonic can realize the localization and depth detection of microdefects on the metal surface owing to broadband surface waves. In this paper, a critical frequency method for measuring the depth of a microdefect is proposed according to the transmission/reflection threshold phenomenon. Based on the principle of photoelastic excitation, a laser ultrasonic testing platform is built and B-scanning images of aluminum alloy samples and the localization of microdefects are obtained by using the platform. Spectral energy distribution of the transmission and reflection waves of a microdefect is analyzed by combining the wavelet transform, and the threshold value θ0 is measured as 1/4 when the transmission/reflection threshold phenomenon occurs. At last, the depth estimation of a microdefect is realized by the critical frequency method. In addition, the influences of sample material and the distance between the excitation points and the detection points are also analyzed. Experimental results show that the critical frequency method based on the laser ultrasonic can realize the depth detection of microdefects on the metal surface, and the transmission/reflection threshold value is independent of the propagation distance and the sample material.