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　telescope　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　deduced.　Besides　higher　measurement　precision,　short　thermal　focus　length　can　be　measured　possibly　by　the　method,　compared　with　other　traditional　measurement　method.