An image simulation method for orthodontics is provided. First, receive a face video and capturing a face image from the face video and find a plurality of face feature points on the face image to determine a mouth area. Then, acquire a tooth image in the mouth area. Define a plurality of tooth image feature points according to the tooth image. Subsequently, compare a plurality of preset tooth model feature points of the stereo tooth model and the tooth image feature points, to adjust the preset tooth model feature points to conform to the tooth image feature points and further to form an adjusted stereo tooth model. Then, project the adjusted stereo tooth model to the face video. An image simulation device for orthodontics is also provided.