After a period of operation,cracks may occur in the material or welding seam of steel structures,so that some repair measures should be adopted to prolong the lifetime of the structure.This paper deals with the fatigue lifetime and reinforcement parameters of cracked steel plates strengthened by carbon fiber-reinforced polymer ( CFRP) .In the investigation,first,a finite element model of the cracked plate strengthened by CFRP is established based on the cohesive force theory,with which the stress intensity factor of the crack is calculated.Then,four groups of fatigue testsare carried out.According to the test data and the Paris formula，the material constants C and n are calculated.Moreover,the calculated stress intensity factor is put in the Paris formula to predict the fatigue lifetime of the specimen.Finally,the stiffness,length and width of CFRP are studied by using the finite element model.The results demonstrate that the stress intensity factor calculated by the finite element model and the Paris formula helps to accurately predict the fatigue lifetime of the cracked plate strengthened by CFRP,and that CFRP is effective in decreasing the stress intensity factor of the crack edge and prolonging the lifetime of steel plates.