Pyrolysis of lignite was conducted at high temperatures from 400 to 700℃ and high pressures of 1 and 3 GPa in a closed system in order to investigate pressure and temperature effects on organic matter maturation. The data indicated that the total yield of liquid hydrocarbons decreased with increasing pressure and the maximum of production was obtained at a higher temperature. The alkanes show more mature characteristics with increasing temperature at a given pressure, conversely, high pressure significantly hinders the thermal evolution of geological organic matter. The experimental results also demonstrated that the relative abundance of high molecular weight hydrocarbons may exist under the conditions of the lower lithosphere, which not only breaks through the traditional concept that alkanes mainly present at stage of "oil window", but also enhances the possibility to find the oil in the high temperature and pressure areas.