Concerning the difficulties such as small measuring range and complicated operation in traditional train speed measure device, a new speed measuring system for dynamic clearance limit measurement of high-speed train is designed based on scanning laser radar. The scanning laser radar fixed in the position of 10 meters from the train. According to the principle of pulsed time-of-flight laser ranging, after the train enters the scan range the scanning laser radar scans the train point by point along the direction of train obtains the body contours. Process measurement data of cars with least square method to obtain the track and direction of the train. The travel distance of the train during two adjacent measurement cycle can be measured through piecewise linear interpolation. Complete the train speed measurement through the above work. Field experiments show that the system is easy to operate with capacity of 600 km/h and the system measurement error is less than 1.2% meets the demand of high-speed train velocity measurement.