This paper presents a method for online measurement of the dimensions of thermal train wheel by one frame of photo-thermal image. A measurement system and its mathematics model is setup. The effect of the spectrum of laser source to restrain thermal background noise is analyzed, then, a special laser is selected. A weighting method of the gray value transform is added into the image process. The high temperature fields of a measured train-wheel are analyzed by use of limited element method. The experiment result shows that the measurement standard deviation of the thermal train-wheel rim is less than 0.55 mm.