The influence of dynamic thermal effect induced by optical pumping of YAG crystal rod in a pulsed solid laser on the wave front of the output laser beam is studied in this paper. A series of time-resolved instantaneous interferograms during and after the pumping are obtained with a Mach-Zehnder interferometer, and three-dimensional time-resolved pictures of the distortion wave surfaces and the curve of thermal focal length of laser rod versus time are gained after the treatmant and calculation for interferograms, and the curve is explained.