The invention provides a device and a method for measuring the thermal lens focal length of a nonlinear crystal. The device comprises nonlinear crystals, an optical resonant cavity, a single frequency laser, a power regulator, a beam splitter, a photoelectric detector, an oscilloscope, a signal generator and a high-voltage amplifier, wherein the nonlinear crystals are put in the minimum waist spot position of the optical resonant cavity; the output light of the single frequency laser is injected into the optical resonant cavity after passing through the power regulator; the beam splitter separates frequency doubled light output by the optical resonant cavity from fundamental frequency light; the fundamental frequency light is injected into the photoelectric detector; the oscilloscope is used for recording to obtain the transmission spectrum of the optical resonant cavity; a low-frequency signal is generated by the signal generator; after being amplified by the high-voltage amplifier, the low-frequency signal is loaded onto piezoelectric ceramics attached onto the endoscope. The offset quantity of the resonant frequency of the optical resonant cavity is obtained through measurement according to the recorded transmission spectrum; the calculation is performed according to a formula to obtain the thermal lens focal length of the nonlinear crystals. The device and the method are simple; the operation is convenient; the measurement result is accurate; the practical value is high.