The invention relates to dynamic measurement of wheel diameters of railway vehicles such as wagons, passenger trains, bullet trains and locomotives in the normal running state on a railway. A device comprises a left wheel pair diameter size measuring box (4), a right wheel pair diameter size measuring box (5), a left steel rail position mark strip (3), a right steel rail position mark strip (6) and at least one wheel sensor (1). The wheel sensors (1) are connected with a digital camera (8), a digital camera (9), a line laser generator (7) and a line laser generator (10) through electrical signals. After detecting that wheels enter the detecting area, the wheel sensors (1) send signals to trigger the digital cameras and the line laser generators to work and then calculate the wheel diameters of the vehicle according to taken photos. By means of the device and method, railway wheel diameters can be accurately and dynamically measured.