A new type water-cooled heat dissipater for multiple high-power thyristors in explosion-proof shell used in coal mine was designed, and then, the numerical computation of the three-dimensional steady-state temperature distributions under different working conditions for cooling core was conducted in order to understand in detail the heat transfer performance. Based on the computation results, the temperature differences and the maximum heat transfer rates were given. These results of the study on the heat dissipater lay a basis for optimising its structure design and guiding its operation.