A servo includes: a servo body, an upper cover fixedly connected to the top of the servo body, a lower cover fixedly connected to the bottom of the servo body, and a direct-current motor, a motor gear and a gear set which are arranged between the servo body and the upper cover and in driving connection with one another in sequence. The servo further comprises a first part and a second part protruding from the first part, and the gear set comprises an output gear located at the second part.