To measure accurately dynamic thermal focus length of laser crystal in solid state laser, a new and accurate method for measuring dynamic thermal focus length by means of the indicator light polarization converted was presented in the paper. A formula of dynamic thermal focus length was established based on imaging theory of geometrical optics. An indicator light was traveled through laser medium which possessed thermal lens effect round trips. The measuring beam was separated from the optical path by a method of polarization converted. Then, the measuring beam was detected by CCD camera. The experiment setup for measuring dynamic thermal focus length of laser crystal was put up and the dynamic thermal focus length of laser crystal end-pumped and side-pumped were measured respectively. The measuring error was analyzed at last. The result showed that the measuring error of dynamic thermal focus length is only 0.8 mm by means of the indicator light polarization conversion, and the error can completely meet the design requirements for laser cavity.