Abstract: The generation of hydrocarbons occurred through a complex physical and chemical reaction process in finite pore space under lithostatic pressure and fluid pressure (PVT-L jointly controlled condition). Most of the hydrous pyrolysis experiments have been carried out with low-pressure and relatively large reaction space due to experimental device limitations. PVT-L jointly controlled experiments and conventional autoclave hydrous pyrolysis experiments have been carried out with immature shale from the Biyang Sag, Nanxiang Basin, and kerogens from the experiments were characterized by FTIR, which expressed four trends: (1) Accompanying aliphatic group cracking, oxygen-containing group shedding and aromatic group condensation, the evolution of kerogen experienced three periods including slow oil generation, fast oil generation and finishing oil generation. (2)FTIR parameters show more extensive aliphatic group cracking for conventional autoclave hydrous pyrolysis experiments, while there is still relatively strong hydrocarbon generation in the PVT-L jointly controlled experiment. (3)The lower aromatization of saturated hydrocarbons in PVT-L jointly controlled experiment may be relevant to higher fluid pressure in a finite space. (4)The slight increase of oxygen level implies the role of water in the hydrocarbon generation process. Different evolutionary characters of kerogen reflected the differences of reaction mechanism, which implied that it is important to choose an appropriate experimental mode in practical research.