In order to solve the low speed in constringency, getting into local minimum easily and the heavy calculating problem of traditional BP NN, an optimized Levenberg-Marquardt(LM)arithmetic is presented. The arithmetic constructed IDS with optimized BP NN based on a successful misuse IDS. For taking advantage of the anomaly IDS and misuse IDS, experiments prove the new IDS has lower rate of false alert and omitting alert, it could recognise unknown attacks rapidly.