Thermal effects of laser diode (LD) end-pumped solid-state laser are important factors, which affect the laser output characteristics and the system performance. The accurate thermal focal length measurement is an important prerequisite for resonator design. The temperature gradient distribution of LD end-pumped solid-state laser is calculated with the heat conduction equation. The phase difference and the Fraunhofer diffraction pattern of the probe laser are obtained. Based on the above theoretical model, a new method by means of Fraunhofer diffraction pattern of probe laser to determine the thermal focal length is presented, which enables real-time on-line measurement. The optical path difference is calculated from the Fraunhofer diffraction pattern of probe laser, and then the thermal focal length is calculated from the optical path difference. The thermal focal length of laser crystal is measured experimentally, and the feasibility of the method is proved by comparing with the experimental values measured by unstable cavity method.