With the continuous increase of the rail vehicle’s speed, the wear of wheel is becoming more and more serious, thus accelerating the changes of wheels’ diameters, which will certainly bring potential safety hazard to the operation of railway. A new laser dynamic measurement method was put forward, and the principles for measuring diameters by using both one and two laser displacement sensors were introduced. The main factors that influence the accuracy for measuring diameter of wheel were theoretically analyzed and simulated. The results show that adopting two laser sensors can greatly reduce diameter measurement errors caused by the positioning errors when the train moves. A system based on this laser method was built and put into use in a railway site. The experimental results show that the measurement accuracy of using two laser transducers is ±0.38 mm(σ), which can meet the needs of site measurements. And the measurement apparatus can be used in the railway site for dynamically measuring the diameters of wheels.