The phase shift phenomenon occurs in the propagation process of surface acoustic waves excited by laser under Thermoelastic effect. The phase variation law of surface wave and the interaction mechanism between surface wave and defect in aluminum cylinder are numerically simulated by finite element method. Numerical results show that the negative phase of the surface wave generated by laser is the strongest, the negative phase moves to the positive phase in the propagation process, and the phase difference gradually decreases. The intensity of the positive and negative phases is roughly equal at about 180°. After that, the positive phase continues to strengthen and the negative phase continues to weaken, and its phase reverses once every time it travels around the circle. The surface reflection wave produced by the defect has the same phase change phenomenon. Through scanning diagram B, it is found that RRcw becomes fuzzy with the increase of defect Angle, and RRccw becomes obvious with the increase of defect Angle. It shows that the length of reflection wave generation time is proportional to the degree of interference. Based on this, a reasonable detection range is proposed for the surface defects of cylindrical materials. The numerical simulation of defects with different depths indicated that the change of depth made the P-wave peak delay in receiving time. The propagation path of P-wave peak was speculated and the corresponding formula was proposed to prove it. The calculated results were in good agreement with the simulated results, and the defect depth information could be quantified by this formula. The above research results provide an effective reference for ultrasonic detection of surface defects of columnar materials.