This paper discusses the limitation of multi-ply feedback neural network algorithm, presents a fast intrusion detection algorithm, i.e. IBP algorithm, based on the modified BP algorithm: adding a momentum in decreasing gradient formula to calculate the cascade weight value of neuron j to neuron i ,adopting alterable learning rate strategy, choosing batch processing sample input while training the neural network. Bigger learning rates η=0.5 and η=0.65 are selected in the improved algorithm and the structure of three-ply neural network is adopted. The simples of input and output are of fifteen dimension and sixteen dimension. The simulation with computer shows that the modified algorithm is superior to the traditional algorithm in constringency speed, susceptiveness to the initial weight value, stabilization in network.