With the widespread introduction of the concept of human-machine collaboration, the relationship between humans and robots is getting closer during production, and the safety issue in the process of human-robot collaboration has become the focus of its development. In this paper, we propose a new method for online detection of human-machine safe distance based on computer vision. The purposed method is built based on a depth camera and Robot Operating System(ROS). When the system is working, the depth camera collects RGB-D images, the color information is used for object tracking which utilizes the YOLO R object detector and multi-target tracking algorithm. And the deep information is then used for calculating the three-dimensional coordinates of the target in the camera coordinate system. After extracting the information from the RGB-D image, the distance between human and machine can be estimated online to send control commands to the robot. The proposed method can be applied to the industrial field to detect the relative position of the human body and the industrial robot online.