Abstract: The gas reservoir of the Quaternary in eastern Qaidam basin is characterized by unconsolidated and un?diagenetic cap rocks with high porosity and permeability. It has unique sealing mechanism and model for formation of large?scale gas reservoir. This paper calculates the displacement pressure difference between cap rock and reservoir，the theoretical sealed gas?column height, and then evaluates the sealing efficiency, obtains the time that biogas broke through the cap rock and the gas seepage flow rate, and reveals the sealing process of the gas reservoir. The results show that the effectiveness of the un?diagenetic cap rock is becoming better with increase of buried depth, and when real gas reservoir’s gas?column height is bigger than the theoretical sealing gas?column height, the gas below it can percolate and be lost through the cap rock. However, the flow process is short, and then the gas reservoir is also in sealing condition. It is concluded that it is dynamic balancing process for the sealing and seepage of gas reservoir, so the stacked reservoir and cap rock can still seal a certain amount of gas by displacement pressure difference. This could be the stacked sealing model for un?diagenetic gas cap rocks