Growth of laser induced damage on the surface of fused silica plays a major role in determining optics lifetime in high power laser systems. In this paper, a femtosecond laser system is applied to create benign mitigation pits to replace growing damage sites; the mitigation mechanism is also explored. The electric filed distribution around mitigation pit is modeled with the finite difference time domain (FDTD) method to determine the optimal mitigation geometry. The shape、size and depth of mitigation pit are controlled by varying energy of femtosecond laser, changing the movement mode of sample stage, in order to achieve the optimal mitigation geometry. The results of laser damage growth test indicate that the rectangular mitigation structure can reduce the light intensity distribution and the damage threshold of mitigation pits is much higher than damaged sites. Furthermore, Energy dispersive spectrometers (EDS) microanalysis technique is used to detect the chemical composition of mitigation pits, and results shows that ultraviolet absorbing defects are removed after mitigation. Reduces of the ultraviolet absorbing defects as well as the local light intensity are key factors to succeed in mitigating growing damage sites.