A UAV flight control system based on STM32F4 and FreeRTOS operating system is designed. Aiming at the problem of large size and high cost of processor ,a set of flight control hardware system consisting of dual micro controllers, various sensors and actuators is designed based on STM32F4 ,achieving the control of attitude and rudder position, serial communication capability for data acquisition and ground communication., and aiming at the problem that the operating system is expensive and the source code is not accessible, the flight control software system is designed based on FreeRTOS, reducing the burden of RAM while ensuring real-time performance. For improving the reliability of the system, dual residual design of the controller and the watchdog module are introduced. The system has been validated by flight trail, which confirms that the system has high stability, high control accuracy and fast response.