Aimed at the lower access efficiency between administrative domains (AD) in NGN multimedia communication, we adopted the coding pattern for telephone number based on ITU-T E.164 protocol and the method in resolving process of DNS, and put forward a routing addressing algorithm for interdomain communication. This algorithm was based on quasi-homo-administrative domain (QHAD). It used H.225 Annex G protocol messages to exchange address templates between superior level and lower level ADs. Therefore, they could get real-time address information. And all level ADs could keep a routing table of all ancestors. When it initiated a call, it applied for resolving an address to the furthest quasihomo- level ancestor directly. So the resolving process could be done in a comparatively smaller domain. By analyzing the algorithm performance and simulating the algorithm model, we find it can reduce the response delay of routing greatly. It can also decrease the using and traffic of communication channels. In addition, it has the same coding pattern for telephone number like PSTN. So it can realize the intercommunication between PSTN network and Internet easily.