A new space-time calculation method for real-time traffic detection is proposed in this paper. The continuous images containing the information of time and space are first gained from the traffic surveillance video by using space-time diagram. Then the track for a period of time is calculated based on the information of vehicles’ edges，shapes，and roadway taking-up rate after the process of edge detection and image segmentation. The video used in this study is captured by the camera mounted in the roadside building. The experimental results show that the method can be effective and accurate to detect the traffic flow for a period of time，and can be applied to real-time traffic monitoring with considerable practical value.