Firstly this paper gives a brief overview about the general network topology discovery method in network management system, then on base of the character of GPON structure and the reference of SNMP protocol, it designs a project which can discover and display the topology of OLT(Optical Line Terminal) and ONU(Optical Network Unit)in passive optical networks, and using C++ language, we write a detection and display module at the network management station. Finally, by use of the simulative receive and send system at the agent side ,it achieves the function which can discover the topology as well as the status of ONU through OLT agent at the network management side.