Recently, multi-rotor unmanned aerial vehicles (UAVs) have grown rapidly in popularity due to their simple structure, low cost and ease of operation. And they have been widely used in both academic research and commercial applications. As a core component of UAV, autopilot is normally used to realize autonomous control and navigation, including platform stabilization, flight trajectory generation, mission planning and so on. In this paper, we present a survey of existing autopilots for multi-rotor UAVs. First, we will introduce background and principle configuration of autopilots. Second, we will compare and analyze several popular autopilots in the market. Besides, several quality control methodologies will be introduced to improve the reliability and robustness of autopilots. Finally, we will discuss possible trend and technologies for autopilots in future.