A method for detecting fault in a next generation network includes: establishing a tool set and a corresponding command set for detecting whether the network is connective and detecting a range of network fault; creating dynamically in a PROXY an address mapping table storing a correspondence relationship between a user identifier userid and address information of a user terminal; and upon occurrence of a fault in the network, obtaining the address information of the user terminal from the address mapping table according to the userid of the user terminal probed currently, executing the set of command according to the address information, and probing connectivity of the network between the PROXY and the user terminal. The problem that the network address transform (NAT) device and the firewall cannot be traversed may be avoided, and the fault point or range may be detected accurately in the NGN where a fault occurs.