Abstract: Based on the macroscale and microscale mechanics method,a research on fatigue damage evolution of welded joint is conducted.The propagation trace of fatigue crack in welded joint is considered to be with a fractal characteristic within certain scale range.By use of fractal geometry theory,a model of fatigue crack propagation is established,in which the fracture mechanics theory is connected with fatigue damage evolution.An expression of fractal effective stress intensity and a equation of fatigue damage evolution are given.A new quantitative method is provided for describing the characteristics on fatigue crack surfaces of welded joint from microscale.