The present invention provides a measurement method of a nonlinear crystal thermal lens focal length. The method comprises the following steps of injecting a little of to-be-measured laser in an optical resonant cavity, scanning the cavity length of the optical resonance cavity, shaping the injected light spot by a lens set to enable the light spot to match a fundamental mode of the resonant cavity, and recording a matching efficiency; increasing the injected light power, locking the optical resonant cavity, and continuing for several minutes; unlocking the optical resonant cavity, scanning the cavity length, and recording the mode matching efficiency of the injected light and the fundamental mode; contrasting the two mode matching efficiencies, and calculating the thermal lens focal length of a nonlinear crystal. The method is convenient to operate, is high in measurement precision, and has a very good practical value.