To gain an in-depth understanding of the thermal effect of the excimer laser on the linewidth narrowing module (LNM), we investigated the influence of the internal heat accumulated in the module on the output spectrum of the system. Through theoretical analysis and experimental verification, we consider the uneven distribution of the refractive index caused by the temperature change of the prism and the influence of different inert gases on the long-term stability of the output laser spectrum. The results show that the laser interacts with the internal components in the LNM to increase the temperature of the optical components and broaden the output spectrum. The inert gas protection system reduces the accumulation of heat inside the LNM through gas flow and maintains the long-term stability of the output spectrum. The accumulation of heat inside the module has a great influence on the spectral purity value, and has little influence on the full width at half maximum. Compared with N₂, the refractive index of He has a smaller change with temperature, which can keep the spectral purity of the output laser spectrum stable for a long time.