A proxy includes a session keep-alive detection converting unit, configured to perform conversion between a first session keep-alive detection message supported by a first session communication peer and a second session keep-alive detection message supported by a second session communication peer. Another proxy includes a session group keep-alive detection processing unit, configured to act as an intermediate agent for the session group keep-alive detection between a session group communication peer and a session communication peer. A method and network system for session keep-alive detection based on two proxies is also provided. The present invention can implement conversion between different session keep-alive detection mechanisms, and perform keep-alive detection on the session group, thus reducing the message traffic in the session keep-alive detection.