Based on the measurement theory of laser medium thermal abberation with interference fringe, a method is proposed to measure the dynamic thermal abberation of slab laser medium. The interference fringe is recorded by charge coupled device (CCD) cameral and then processed by computer. With analysis of the interference fringe, a simple and rapid algorithm to extract the center of interference fringe is proposed. The dynamic thermal abberation of slab laser medium in pumping process by analyzing and calculating the movement of interference fringes is obtained, which paves the way to dynamically compensating the thermal effect of the laser medium. In experiments, N31 phosphate laser glass is used as the sample, and the interior temperature distribution during heating procedure is obtained. The relative error is about 3%, and the validity of the method is proved.