The forward and backward generation power flow algorithm has the advantages of fast calculation and good convergence. It is especially suitable for the power flow calculation of radial distribution network, but there are few studies about the calculation of distribution network fault flow. the calculation principle of forward backward power flow algorithm is analyzed and the mathematical model of the iterative process is constructed for the fault distribution network. The convergence criterion of the algorithm is proposed and the phenomenon of non convergence is changed by adding relaxation factor. The validity and accuracy of the algorithm are verified by data simulation in MATLAB.