A discuss on the thermal-lens effect of three nonlinear optical crystals of great commercial value——LBO, KTP and BBO was given in the paper. The curve that the focal length of three crystals mentioned above varying with the spot size of fundamental frequency delivered on the face of the nonlinear optical crystals was also given. At the same time, KTP thermal focal length was calculated 852 mm on the condition that the incident spot size w=368 μm according to 20 W output green laser which used KTP crystal as its double frequency crystal, which agrees well with the experimental data.