Lithium-sulfur (Li-S) batteries have emerged as one of the most hopeful alternatives for energy storage systems. However, the commercialization of Li-S batteries is still confronted with enormous hurdles. The poor conductivity of sulfur cathodes induces sluggish redox kinetics. The shuttling of polysulfides incurs the heavy failure of electroactive substances. Tremendous efforts in experiments to seek efficient catalysts have achieved significant success. Unfortunately, the understanding of the underlying catalytic mechanisms is not very detailed due to the complicated multistep conversion reactions in Li-S batteries. In this review, we aim to give valuable insights into the connection between the catalyst activities and the structures based on theoretical calculations, which will lead the catalyst design towards high-performance Li-S batteries. This review first introduces the current advances and issues of Li-S batteries. Then we discuss the electronic structure calculations of catalysts. Besides, the relevant calculations of binding energies and Gibbs free energies are presented. Moreover, we discuss lithium-ion diffusion energy barriers and Li2S decomposition energy barriers. Finally, a Conclusions and Outlook section is provided in this review. It is found that calculations facilitate the understanding of the catalytic conversion mechanisms of sulfur species, accelerating the development of advanced catalysts for Li-S batteries.