The model of the conductivity of the working fluid and the surface roughness of the work piece of the Wire cut Electrical Discharge Machining (WEDM) is studied. Because of the test data too few, it cannot be creating a reliable model. Based on the original data, a way is proposed that using BP neural network to expand the data and to establish a prediction model. The results show that the average error between the predicted value and expected value is -0.001 7, the absolute error is 0.008 5, the mean square error is 8.169 5×10-5 and the standard deviation is 0.009 0, which is of high accuracy. The application of the BP neural network model can solve the problem that the test data was too few to be creating a reliable model, which can provide a reliable method for the processing test data and modeling of the WEDM.