Due to the geometry irregularities of specimens and the inhomogeneous of materials, micro-crack is usually generated by abrupt change of local stress. Traditional detecting techniques using ultrasound, such as pitch-catch method, are difficult to distinguish the micro-crack effectively, because the size of the cracks are so small that they are often hidden from ultrasonic detection by a stronger scattering from the very same structural imperfection, such as surface protuberance, surface grooves, corrosion pits, etc. The laser ultrasonic detecting system is built to detect the surface irregularities based on one-sided pitch-catch method. The laser line source is used to excite ultrasound and the optical deflection beam method is used to detect ultrasound. The traditional pitch-catch method, scanning laser source method and the scanning heating laser source method are used to detect the surface irregularities of specimen which contains a surface protuberance and a surface breaking micro-crack. The pitch-catch method requires high signal to noise ratio of signals and the reliability is poor. The scanning laser source method can detect both one but can not distinguish the crack from them. Based on the characteristics that the micro-crack can get closed partially after illuminated by heating laser, the scanning heating laser method can recognize it efficiently. The finite element method (FEM) is used to calculate the transmitted surface acoustic waves (SAW) which are influenced by different sizes of surface breaking cracks.