The velocity measurement algorithm based on vision is widely used in unmanned aerial vehicle navigation. Under uneven illumination intensity distribution, the traditional Lucas–Kanade (LK) optical flow (OF) algorithm has problems arising from low computational accuracy and poor adaptability. To solve these problems, we propose a monocular vision integrated velocity measurement system based on the square-root cubature Kalman filter (SRCKF). The LK OF and the optimized oriented FAST and rotated BRIEF (ORB) algorithms are used to process the visual information obtained using a camera. The SRCKF algorithm is tasked with fusing the LK OF and optimized ORB information, thereby improving the accuracy of velocity and alleviating the sensitivity of the LK OF to variations in illumination conditions. Finally, an outdoor unmanned aerial vehicle flight test was undertaken. The experimental results show that the proposed method provides an accurate measurement of the velocity in variable illumination environments.