A reverse transmission calculation of self adaptive learning rate with momentum gradient reduction is described in the paper against the problems of slow convergence and easy trapping into smallest spot . The calculation has made the BP net upgrade the rate and stability of learning .In the paper, an improved BP networks are used in the fault diagnosis of distribution network.And an improved BP networks are trained as a classifier of the distribution network’s fault. It, sets up a simple system of fault diagnosis about distribution network by the Visual B and Matlab,which has made an effective fault diagnosis of distribution network.