In order to solve the problem of real-time attitude measurement of quad-rotor UAVs in flight and improve the accuracy of attitude estimation, a gradient-descent attitude estimation system based on STM32 is designed.The system takes STM32F427 microprocessor as the main controller, MPU6000 and LSM303D as the attitude sensor, and realizes data acquisition of the attitude sensor and attitude calculation.The accelerometer and magnetometer are pre-processed by the system, and the error quaternion is estimated by using the method of gradient descent filtering, and thus the integral error of the gyroscope is corrected and the accurate estimation of the attitude angle is realized.Finally, a comparison is made with the attitude angle of Pixhawk flight control system by using the established flight experiment platform.The experimental results show that the attitude estimation system using the gradient descent method can estimate the attitude of UAVs effectively and stably, which can meet the flight requirements of quad-rotor UAVs.