A thermal control system (TCS) based on the resistance heating method is designed for the High Energy Detector (HED) on the Hard X-ray Modulation Telescope (HXMT). The ground-based experiments of the active thermal control for the HED with the TCS are performed in the ambient temperature range from −15 to 20°C by utilizing the pulse width to monitor the interior temperature of a NaI(Tl) crystal. Experimental results show that the NaI(Tl) crystal's interior temperature is from 17.4 to 21.7°C when the temperature of the PMT shell is controlled within (20±3)°C with the TCS in the interesting temperature range, and the energy resolution of the HED is maintained at 16.2% @122 keV, only a little worse than that of 16.0% obtained at 20°C. The average power consumption of the TCS for the HED with a low-emissivity shell is about 4.3 W, which is consistent with the simulation.