The purpose of this paper is to find a simple and effective method to evaluate the damage tolerance of complex structure. Aiming at the problem of evaluating fatigue life of complex structure repaired in aviation engineering, a simple and effective method was put forward to obtain the stress intensity factor. Combined with damage tolerance analysis method, the fatigue life and the residual strength of the repaired structure with cracks in the flange of a frame floor of a frame are analyzed. A simplified analysis model is built according to the loading characteristics of fuselage floor beam structure, the stress intensity factor of crack tip under different crack length was calculated, and then the dimensionless stress intensity factor is determined by the relationship between structural boundary load and stress intensity factor. According to the damage tolerance analysis method program, the lifetime of the structure from the initial crack to the critical length and the residual strength of the structure under the length of each crack are calculated. The analysis shows that the fatigue life and residual strength of repaired structures meet the design requirements of damage tolerance. The damage tolerance analysis procedure and method presented in this paper can be applied to evaluate the damage tolerance of similar structures in engineering.