The invention provides a method for measuring thermal focal length of a laser, comprising the steps of: constructing an attachment lens-mounted resonant cavity having astigmatism, and for an astigmatic cavity with parameters being determined, leading meridian plane waist and sagittal plane waist on certain branch arm to be equal in correspondence to the focal length-determined attachment lens; calculating a corresponding relationship between the focal length of the attachment lens and the length of the branch arm when the meridian plane waist and the sagittal plane waist on the certain brancharm are equal; using a gain medium to replace the attachment lens in the resonant cavity in order to output laser, observing the spot pattern of the output laser through a detection device, and recording a pumping power at the moment the thermal focal length of the gain medium and the focal length of the attachment lens are equal in case that spot patterns of the output laser at any two points are both circular; and obtaining the thermal focal length of the laser based on the one-by-one corresponding relationship between the length of the branch arm and the focal length of the attachment lensand on the one-by-one corresponding relationship between the length of the branch arm and the pumping power. The method is simple in device, convenient in operation, low in possibility of damaging laser gain medium and small in measurement error.