Underground structures are normally located in highly-confined and congested spaces, which may lead to severe gas explosion accidents, with significant human and economic losses. For accurately evaluating the consequences of these explosions, a variety of influencing factors need to be considered, including concentrations of gas mixtures, vent conditions, obstacles, and ignition features. Moreover, a good review on these influencing factors is important for a better understanding of explosion behavior, e.g., the deflagration to detonation phenomenon. In this study, some critical influencing factors for gas explosions in underground or confined spaces are investigated, and the effects of the factors on such gas explosions are examined. The results are discussed, along with findings from literature. The present study provides a reference for future studies on safety management and consequence mitigation for underground gas explosions.