Abstract: By the end of 2001, 21 large gas fields had been discovered in China, and their cumulative explored reserves of gas in place reached 17953×108 m3, taking up 59.80% of China＇s total gas reserves. Gas molecules are small, hard to be adsorbed and ready to diffuse. Large early-formed gas fields will lose their quantity and become small or medium-sized ones or even be depleted completely owing to long-continued gas diffusion, if no gas source replenishes the supply. Therefore, the formation of gas accumulations in the late stage is one of the fundamental conditions for the formation of large gas fields. Basins in China are polycyclic in nature. The tectonic cycle of succeeding basin usually reduces or damages the reserves and preservation conditions of gas accumulations in the preceding basins. So, the formation of gas accumulations in the late stage can avoid this defect and is favorable for the formation of large gas fields. The period of peak gas generation of large gas fields resulting from late-stage accumulation is synchronous or nearly synchronous with the period of accumulation, and their source and reservoir rocks are younger. Sometimes, the peak generation period is earlier than the period of accumulation and their source and reservoir rocks are older.