A method for an online game system triggering an IM operation includes: setting a game user identification and a corresponding IM user identification in an online game server; detecting, by an online game client, whether an IM operation instruction regarding a game user identification is received, and querying for the IM user identification corresponding to the game user identification if it is determined that the IM operation instruction is received; and, invoking, by the online game client, an operation interface of an IM client, the IM user identification being taken as an invoking parameter and the operation interface corresponding to the IM operation instruction and being to trigger a corresponding IM operation. An online game system is also provided. Thereby, the IM operation can be triggered in an online game, and thus the process for triggering the IM operation is simplified and the IM client's system resources are saved.