Aiming at the problems that the time-varying branch fault rate, the uncertainty of harmonic content and the correlation of random variables affect the accurate calculation of limit line loss, combined with the calculation requirements of flexible interconnected distribution network, a limit line loss calculation method considering multiple uncertain factors is proposed in this paper. Firstly, the improved three-stage Weibull distribution is used to fit the branch failure rate, and the probability distribution model of branch failure rate is established. Combined with the full probability formula, the loss probability distribution considering the time-varying of branch failure rate is solved. Then, the correlation model of the harmonic content of the converter with the transmission power is established to solve the problem that the harmonic can't reflect the influence of harmonic fluctuation on the standard deviation of line loss. Secondly, in order to quantify the influence of the correlation of random variables on the limit line loss, the Nataf inverse transform is introduced to improve the three-point estimation method, and combined with the failure rate probability model and harmonic content correlation model, the calculation method of limit line loss under multiple uncertain factors is constructed. Finally, an example of improved IEEE 33 node system shows that the proposed algorithm has high accuracy and fast calculation speed.