To understand the activity range and characteristics of hydrothermal fluids in the Songliao basin and analyze their influence on the porosity- permeability of the shale reservoir,as well as their relationship with present- day overpressure, this study focuses on the Qingshankou Formation shale in the Gulongsag. Our research combines microscopic and scanning electron microscope observations, electron probe measurement, XRD analyses of clay minerals, homogenization temperature and Laser Raman spectroscopy of fluid inclusions, vitrinitereflectance, rock pyrolysis, and carbon and oxygen isotope analysis of carbonate cement. We integrate these findings with previous research results. Our results show that hydrothermal fluids, associated with tectonothermal events and magmatic activity, have significantly impacted the shale reservoir. This is evidenced by anomalies invitrinite reflectance and rock pyrolysis parameters, accelerated transformation rates of authigenic clay minerals, and a higher fluid inclusion homogenization temperature than expected based on maximum buried depth. The δ18O values of carbonate cement and δ34S values of authigenic pyrite are consistent with those of magmatic rocks,further supporting the influence of hydrothermal fluids. We identified a typical hydrothermal mineral assemblage in the shale, suggesting that deep hydrothermal fluid activity primarily occurred below 2000 m. The zones of high porosity and permeability in the Qingshankou shale coincide with zones of hydrothermal fluid activity and overpressure development. The intrusion of hydrothermal fluids, rich in inorganic CO2, inhibited carbonate cementationto some extent and promoted the dissolution of soluble minerals, leading to the formation of secondary pores and improving the physical properties of the shale reservoirs. Meanwhile, hydrothermal activity accelerated the thermal maturation of organic matter,enhancing hydrocarbon generation from the source rocks and increasing its hydrocarbon production capacity. We propose that the abnormal pressure in the Gulong sag is mainly due to hydrocarbon- generating expansion, with hydrothermal activity and its associated heating effect also contributing to the maintenance of overpressure.