Aiming at the copyright protection problem of 3D medical images and the simultaneous expansion problem of watermark storage capacity caused by the increase of the number of images to be protected， a robust zero-watermarking algorithm based on ray-casting sampling and polar complex exponential moment was proposed. Firstly， a sampling algorithm based on ray-casting was proposed to sample the features of 3D medical images composed of multiple sequences of 2D medical images and describe these features in 2D image space. Secondly， a robust zero-watermarking algorithm for 3D medical images was proposed. In the algorithm， three 2D feature images of coronal， sagittal planes and cross section of the 3D medical image were obtained by ray-casting sampling， and the three 2D feature images were transformed by polar complex exponential to obtain the quaternion orthogonal moment. Finally， the zero-watermarking information was constructed by using the quadratic orthogonal moment and Logistic chaotic encryption. Simulation results show that the proposed algorithm can maintain the bit correctness rate of zero-watermarking extraction above 0.920 0 under various common image processing attacks and geometric attacks； the watermark storage capacity of the proposed algorithm can be improved with the increase of the volume of 3D medical image data， and the storage capacity of the proposed algorithm has been improved by 93.75% at least compared to the other 2D medical image zero-watermarking algorithms for comparison.