A method, a system, a mobile terminal for adjusting focal length of the camera includes monitoring, by the proximity sensor arranged on multi-functional keys of a side of the mobile terminal, movement direction, movement distance, and movement time of fingers in real-time, and uploading the movement direction, the movement distance, and the movement time to an application processor of the camera. An application processor of the camera provides analyzed data of the movement direction, the movement distance, and the movement time, and adjusts the focal length of the camera according to a result of the analyzed data.