Abstract: The Longmaxi black shales of the Early Silurian in Sichuan basin is mainly deposited in shallow-deep water shelf sedimentary environment, with big thickness and wide distribution. High-output intervals of the shale gas wells in Longmaxi formation are all in abnormal high pressure. The study suggests that undercompaction pressurization, minerals digenesis, tectonic compression and thermal evolution of organic matter are considered to be the main factors causing the abnormal high pressure. The abnormal high pressure has many effects on the shale gas reservoirs. Firstly, it can restrain the thermal evolution of source rocks, and raise vitrinite reflectance lower limit for gas generation threshold. Secondly, it can slow down the digenesis of compaction for keeping higher porosity and permeability, and form secondary pores and micro-fractures by organic acids corrosion and rock effective stress decrease, hence improve the ability of reservoir seepage and put forward higher requirements for the fracturing modification. Thirdly, the content of absorption gas increases quickly with formation pressure raise, which makes the gas-bearing of the shale reservoir better. In short，comprehensive influences of abnormal high pressure are favorable for enrichment of shale gas