Abstract: The deep part and middle lower parts of Songliao Basin develop in different evolution stages of basin.There are differences of gas sealing conditions between deep and middle lower part of the north of Songliao Basin. The thickness and distribution of K₁d₂ and K₁q₁₊₂ mudstone caprock are smaller than those of K₁q_n and K₁n₁₊₂ mudstone caprock . The space matching relation between K₁d and K₁q₁₊₂ mudstone caprock and their sourcerock is worse than that between K₁q_n, K₁n₁₊₂ mudstone caprock and their sourcerock . Except for capillary sealing ability, K_1qn and K₁n₁₊₂ mudstone caprock own pressure sealing and restraining hydrocarbon concentration sealing ability. Except for capillary sealing ability, K₁d₂ mudstone caprock owns replacing hydrocarbon concentration sealing ability . The time matching relation between K₁q_n, K₁n₁₊₂ mudstone caprocks and themselves sourcerocks is better than that between K₁d₂, K₁q₁₊₂ mudstone caprock and K₁sh+yc sourcerock. They are main causes of better accumulation degree and distribution of middle and lower combination than that of deep combination.