A servomotor includes a control circuit, an electric motor, a harmonic drive and a sensor. The control circuit is connected to the electric motor and used to control the electric motor. The harmonic drive includes an outer casing, a wave generator, a flex spline and a circular spline. The wave generator is driven by the electric motor. The flex spline is sleeved on an exterior of the wave generator and located within the circular spline and engages with the circular spline. The flex spline is connected with an output member that is used to output power, and a post is arranged along a rotation axis of the output member. The sensor is arranged within the outer casing and used to detect an angular displacement of the post. A method for controlling the servomotor is also provided.