A Nd³⁺:YAG laser is pumped by a fiber-coupled and GRIN lens-focused diode-laser and the focal length of the equivalent thermal lens is measured by using unstable cavity method. Meanwhile, the theoretical model describing the thermal lens is extended to the situation that the pump light radius in the gain medium varies with distance. The calculated focal length of the thermal lens is inversely proportional to the pump power which is in conformity with the experimental result. The difference between them is also discussed.