When an accident occurs in a nuclear power plant, the main control room (MCR) is isolated while initiating emergency measures. To keep the indoor temperature from increasing to dangerous levels, heating loads need to be mitigated by heavy concrete insulation, on which rectangular fins are installed to improve the heat transfer. However, the effect of optimized fins for the thermal environment has not been investigated. In this study, a parametric model based on COMSOL Multiphysics is proposed to describe the dynamic heat transfer process of a fin-concrete heat sink and validated by an experiment on a 1200 mm × 1200 mm × 610 mm module. A COBYLA algorithm has been implemented to optimize the fin parameters combination with different steel consumption. Results show that both steel consumption and fin structure parameters need to be taken into consideration when optimizing. The optimal heat transfer increases by 2.84%, 7.33%, and 9.26%. The maximum indoor temperature is 0.77, 1.85, and 2.39 °C lower than the prototype when the steel consumption is 100, 150, and 200 kg/m2, respectively. Furthermore, the heat storage will improve as increasing the steel consumption if the fin structure is optimized. And it does not necessarily so without optimization. Finally, the optimization potential increases with the decrease in fins space, the increase in fin height, and the increase in fin thickness. In the case of only taking reducing the maximum indoor temperature into account, reducing fins space and increasing fin height are the superior choices for the optimal design of heat sink.