The invention discloses a solid laser thermal focal length measurement method and device. By utilizing a law that a stable area of a solid laser moves along with curvature change of a spherical deformation mirror, a relationship between laser output power and pumping power in a certain range is measured to gradually reduce a position of the stable area, and finally, a thermal lens focal distance under each power is calculated through a linear relationship between thermal lens focal power and the pumping power. According to the method and device, an extra light path is not required and the mechanical structure of a resonant cavity does not need to be changed during the operation on one hand, so that the operation is more convenient when being compared with the traditional method, and the focal power change range of thermal lenses can be accurately measured by utilizing a two-step method of stable area maps and measurement can be realized under practical operation states of laser crystals on the other hand, so that the reliability of measurement results is further improved.