Based on the study of seal mechanisms of caprock to each phase of oil or gas and their influence factors, this paper sisterly studied seal characteristics of K_1n~1 mudstone caprock to each phase of oil or gas in Wuerxun depression. This paper considered mudstone caprock of K_1n~1 to have best direct ability of capillary seal to oil or gas in free phase, to have best indirect ability of abnormal pore fluid pressure to oil or gas in free phase in the middle of depression and have better and diddle in directly.