Methods for implementing a message service based on IP Multimedia Subsystem (IMS) are disclosed in embodiments of the invention. One of the methods includes: determining a network domain of a called subscriber according to information in a message service of a calling subscriber; conducting the message service according to a type of the network domain of the called subscriber. Systems for implementing a message service based on IMS, IMS-based Serving-Call Session Control Function (S-CSCF), and IP-Short Message-Gateway are disclosed in the embodiments of the invention. The invention facilitate the network domain of calling side to select different processing procedures for the called subscriber information in different network domains by deciding the network domain of called subscriber, which makes the interworking procedure between an IMS-based message and a non-IMS-based message service clearer and the interworking procedure between an IMS-based message and the traditional short message more perfect.