Disclosed is a method of activating a mobile terminal from mobile network side and a mobile application gateway system. The method comprises: receiving, by a mobile application gateway, a request for activating the mobile terminal, the request including an unique identification number of the mobile terminal; querying, by the mobile application gateway, a HLR for address of a SGSN where the mobile terminal is located; returning, by the HLR, the queried address of SGSN to the mobile application gateway; sending, by the mobile application gateway, the request for activating the mobile terminal to the SGSN; issuing, by the SGSN, a request to the mobile terminal, requiring the mobile terminal to issue a mobile terminal initiated PDP context activation request. The method enables an instant messaging user at a mobile terminal to automatically open an IP connection if there is any message to be received after the IP connection is closed.