The interference phenomenon of Lamb waves at overlap joints are researched by the numerical simulation and theoretical method. The numerical method is used to simulate the interference occurred when the Lamb waves S₀ and A₀ mode (at an exciting frequency of 500 kHz) propagate through welds on a 2 mm-thick stainless steel plate. It is found that the waves reflected from the end edge will interfere with the waves reflected from the front edge. The interference was lead to the reflection coefficients change with the weld width periodically. In addition,Lamb waves with different frequency have different interference on the same overlap joint. The interference is observed by simulation. The Lamb waves occur increase interference when the phase difference between two waves is odd times of half wavelength. Moreover,the Lamb waves occur destructive interference when the phase difference between two waves is even times of half wavelength.