Abstract: To solve the problem of crack detection for high-pier and long-span bridges, the bridge crack detection system using quadrotor was designed. The whole system includes three parts, which are unmanned aerial vehicle(UAV) system, camera system and digital image processing. By analyzing the characteristics of the bridge detection and dynamic features of quadrotor, a hardware platform was built with STM32F427 as the main control board, which met the requirements for flight power, flight control and safety. In the camera system, image quality was improved through camera correction; auxiliary information for crack automatic measurement was provided through calibration of pixel physical size and object distance measurement. In addition, a real-time image transmission scheme was designed. In digital image processing, based on noise characteristics of bridge cracks, crack binary image was acquired after median filtering, edge enhancement, threshold segmentation and morphology operations. Then, cracks could be measured automatically using improved "rectangular box method". The UAV bridge detection system presented in this paper provided a feasible scheme for solving the detection problem of cracks in long-span and high-pier bridges, and offers technical support to improve the efficiency and accuracy of bridge detection.