1. A method for automatically playing an expression on a virtual image, comprising steps of: A: capturing a video image related to a game player in a game, and determining 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 facial features from the facial image, and obtaining 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 a corresponding facial expression according to the motion track of the facial feature which is obtained in step B, and automatically playing the determined facial expression on a virtual image of the game player.