A new method to measure dynamic thermal focus length by reference light is put forward on the basis of telescope systems operating principle,which uses keplerian telesoope consisted of an accessorial lens inserted in optical path and thermal lens.According to the relation of wavelength between reference light and Nd:YAG laser,thermal focus length of Nd:YAG laser is decluced.Besides higher measurement precision,short thermal focus length can be measured possibly by the method,compared with other traditional measurement method.