1. A method for automatically playing an expression on a virtual image, comprising steps of: A: capturing, by a machine, a video image related to a game player in a game, and determining, by the machine, whether the captured video image contains a facial image of the game player, and if so, executing a step B; otherwise, returning to the step A; B: extracting, by the machine, facial features from the facial image, and obtaining, by the machine, a motion track of each of the extracted facial features by motion directions of the extracted facial features that are sequentially acquired for N times; wherein N is greater than or equal to 2; and C: determining, by the machine, a corresponding facial expression according to the motion track of the facial feature which is obtained in step B, and automatically playing, by the machine, the determined facial expression on a virtual image of the game player; and further comprising: obtaining, by the machine, a set of correspondence relations between facial expressions and common facial motion tracks through pre-training; wherein, the step C comprises: C1: searching a correspondence relation from the set of correspondence relations, wherein the common facial motion tracks of the correspondence relation are related to motion tracks of the extracted facial features; and if the correspondence relation is searched, then executing a step C2; C2: automatically playing the facial expression in the searched correspondence relation on the virtual image of the game player; wherein, in the step C1, the fact that the common facial motion tracks of the correspondence relation are related to motion tracks of the extracted facial features comprises that: a match between the motion track of each of the extracted facial features and a respective one of the common facial motion tracks is larger than or equal to a match value corresponding to the extracted facial features.