Compared with conventional natural gas, shale gas has special occurrence forms, and the content of shale gas in different forms varies greatly. To evaluate the difference of shale gas occurrence forms and their gas- bearing capacity with different thermal maturities under different sedimentary environments, a comparative study was conducted on 15 low- mature shale samples from Yanchang Formation in Ordos Basin and 15 high- mature shale samples from Longmaxi Formation in Sichuan Basin. Based on the laboratory experiments including vitrinite reflectance measurement, TOC content analysis, field emission scanning electron microscopy observation, low temperature nitrogen adsorption and methane isothermal adsorption experiments, the occurrence mechanism of methane in pore space were qualitatively analyzed, and the content of shale gas in different occurrence forms was quantitatively calculated. Combined with the thermal evolution degree, pore characteristics and gas content, a dynamic evolution model of shale gas occurrence forms was established. In different stages of thermal evolution, the occurrence forms of shale gas and their content are quite different. For low- mature Yanchang shales, the content of adsorbed gas accounts for 58%, free gas accounts for 32% and dissolved gas accounts for 10%, which shows the characteristics of "predominant adsorbed gas, secondary free gas and non- negligible dissolved gas", however, for high- mature Longmaxi shales, the content of free gas accounts for 51%, adsorbed gas accounts for 48%, and dissolved gas accounts for only 1%, which is characterized by “co- dominant free gas and adsorbed gas, and negligible dissolved gas”. With the increase of thermal maturity, the generated shale gas first saturates the adsorption and partial dissolution, then the excess gas stores in the form of free gas, and finally the adsorption gas and free gas are in a dynamic equilibrium before tectonic and production, which corresponds to four stages of shale gas reservoir formation, namely, adsorption stage, pore filling stage, fracture filling stage and accumulation stage. Combined with the production characteristics of shale gas reservoirs, the shale gas reservoir of Yanchang Formation is dominated by adsorbed gas, with the features of lower overall gas production, lower initial productivity, and relatively longer stable production period; while the shale gas reservoir of Longmaxi Formation is dominated by both adsorbed gas and free gas, with the features of higher overall gas production, larger initial productivity, relatively longer stable production period and greater gas reservoir potential.