In order to solve the difficult problem of unmanned air vehicle(UAV) target detection in visible light images under complex sky background, this paper proposes a UAV target detection method based on frequency domain transform. First, the B-channel in the image LAB space is used to extract the sky and cloud boundary images, and then the image feature channel is used to construct a quaternion function. Secondly, Fourier transform is performed on the quaternion function to extract the amplitude spectrum and phase spectrum, then the amplitude spectrum image is subjected to multi-scale decomposition using wavelet transform in the frequency domain, the amplitude spectrum image of each scale and the phase spectrum image are combined by inverse Fourier transform, and the evaluation function is used to obtain the best scale image. Finally, the best-scale image and the boundary image are normalized to make a difference to obtain the final detection result. Experimental results show that the algorithm can effectively detect UAV targets under complex cloud background.