In order to realize early design of constant-temperature water jacket of lithography lens, a downscaling model test for constant-temperature water jacket is designed with similarity principle, and the method of this downscaling model test design is proved to be correct by numerical simulation. First, governing equations of heat transfer are given based on structure of the constant-temperature water jacket. Then, the governing equations are analyzed, and the similarity criteria of model test design under steady-state heat transfer are derived. Thirdly, a 1:4 downscaling model of a constant-temperature water jacket of lithography lens is designed with the similarity criteria. Finally, thermal distribution in the 1:4 downscaling model and its prototype are analyzed by numerical simulation, and results show that the thermal distribution between them are similar to each other. It can be concluded that the downscaling model test design for constant-temperature water jacket of lithography lens should meet the criteria as follows: geometric similarity, Reynolds number similarity, Prandtl number similarity, Biot number similarity and boundary conditions similarity.