The invention provides a method for measuring an absorption coefficient of a nonlinear crystal. The method comprises the following steps: a little to-be-measured fundamental frequency light is injected into an optical resonator, a proper lens group is selected, the spot of injected light and the spot of a fundamental mode of the optical resonator are enabled to be identical in size and superposed completely, and pattern matching efficiency at the moment is recorded; the injection power is increased, the temperature of the nonlinear crystal is regulated until phase matching is realized, the length of the optical resonator is locked, the output power of frequency-multiplied light is recorded, the optical resonator is unlocked after waiting for a few minutes, and the pattern matching efficiency is recorded quickly; the spot size of the fundamental mode of the optical resonator is calculated according to a measurement result of the pattern matching efficiency; the focal length of a thermal lens is calculated according to the spot size, and the fundamental frequency light absorption coefficient of the crystal is deduced reversely; the light injection power is changed, the steps are repeated, and the absorption coefficients of the crystal under different injection power are measured.