The security of quantum key distribution(QKD)networks based on weak coherent pulses and optical node is analyzed.In this QKD network,an eavesdropper can launch a path-attack on the quantum channel to eavesdrop quantum information. Path-attack will not change quantum key generation rate and not destroy the photon number distribution in a weak coherent pulse,moreover,it can not be detected under the high quantum channel loss. The result indicates that even the decoy states skills cannot detect this kind of threat,also this threat is proportional to the average photon number. Therefore,the influence of the path-attack must be considered when designing the quantum key distribution networks.