In order to study the influence of fly ash on the chloride corrosion in flexurally cracked reinforced concrete, several concrete beams with the mass ratio of fly ash to cementitious material of 0%, 15% and 30% were self anchored in a three point bending mode. Under the sustained flexural loads, these beams with different cracks were immersed in 5% NaCl solution under the condition of dry wet cycles. The results show that under the dry wet cycles, the crack depth at convection region in the flexurally cracked concrete is about 1020mm, which increases gradually with the increase of crack width. The resistance of chloride permeability in concrete improves with the increase of crack width. More than 15% of fly ash content can effectively improve the resistance of chloride permeability of cracked concrete, and its equivalent chloride diffusion coefficient may reduce to 1/8~1/6 of that of ordinary concrete. For the chloride diffusion in cracked concrete, there is an exponential relationship between the damage coefficient, crack width and fly ash content.