A method of sending reset message sequence number is provided. During a connected state handoff of an access terminal (AT) between a source access network (AN) and a target AN, the source AN sends a reset message sequence number of the AT to the target AN. The source AN adds the reset message sequence number of the AT to the message sent to the target AN. Thereby, the problem in the existing systems that the source AN cannot send the reset message sequence number of the AT to the target AN and, thus, the target AN cannot perform a reset process on the AT when the AT is switched from the source AN to the target AN, is solved. An access network (AN) and a communication system are also provided.