In this article, the physicochemical characteristics of the oxidation zone, the reduction zone, and the destructive distillation and dry zone in the process of underground coal gasification (UCG) were explained. The effect of such major factors as temperature, coal type, water-inrush or -intake rate, the quantity and quality of wind blasting, the thickness of coal seams, operational pressure, the length, and the section of gasification gallery on the quality of the underground gas and their interrelationship were discussed. Research showed that the temperature conditions determined the underground gas compositions; the appropriate water-inrush or -intake rate was conducive to the improvement in gas heat value; the properties of the gasification agent had an obvious effect on the compositions and heat value of the product gas. Under the cyclically changing pressure, heat losses decreased by 60%, with the heat efficiency and gasification efficiency being 1.4 times and 2 times those of constant pressure, respectively. The test research further proved that the underground gasifier with a long channel and a big cross-section, to a large extent, improved the combustion-gasification conditions.