The invention provides a motion object detection method and device. The method comprises the steps that in a compressed domain of video streaming, motion vector prediction is performed on pixel blocks of an intra-frame code in a current frame of image, so that motion vectors of the pixel blocks of the intra-frame code are obtained, wherein each frame of image in the video steaming is divided into a set number of pixel blocks according to setting rules in advance, and the sizes of all frames of images are equal; according to the frequency that motion vectors corresponding to pixel blocks located at the same position are non-vanishing vectors in the current frame of image and the continuous M frames of images before the current frame of image, all non-vanishing vectors in the current frame of image are determined; according to all the determined non-vanishing vectors, a motion area in the current frame of image is determined; the motion area is mapped into a pixel domain, in the motion area, preset targets are detected based on the pixel domain target detection algorithm, and the detected preset targets are indicated and then output. According to the motion object detection method and device, the accuracy of target detection can be improved, and the speed of target detection can be increased.