Abstract: The Jurassic in Yinan structural zone with a buried depth over 3000m is in an abnormal high pressure system and gas accumulations develop in the Lower Jurassic. Vertically, the Jurassic has well developed combination of reservoir and seal with good sealing capacity; laterally, the boundary faults also have good sealing capacity. Logging data of interval transit time of mudstone and analysis of the real porosity of mudstone reveal that the formations are normally compacted and have no significant impacts on abnormal high pressure. Generation of gaseous hydrocarbons enlarges the volume of pore fluid in the strata, transform of feldspar to illite enlarges the solid volume of the strata, while structural uplift makes the strata maintaining the original formation pressure. Therefore, the main factors resulting in the abnormal high pressure in the Jurassic in Yinan structural zone are structural uplifting, generation of gaseous hydrocarbons and transform of feldspar to illite.