High clay content, a variety of pore forms, and complicated fluid occurrence are characteristics of shale reservoirs. Shale's electrical conductivity is influenced by many factors, including its mineral composition, fluid type, physical properties and maturity, which leads to the weak correlation between electrical conductivity and oil content and physical properties. Therefore, it is critical to understand the conductive mechanism and influence factors of the electrical properties of shale. In this paper, the mature lacustrine shale of the first member of the Qingshankou Formation (Qing1 Member) in Changling Sag, south Songliao Basin is taken as an example. Using sealed coring, Two-dimensional nuclear magnetic resonance, field emission scanning electron microscopy, TOC, and X-ray Diffraction tests, combined with electrical logging curves, the impacts of fluid occurrence, physical properties and mineral composition on the electrical properties of the shale were analyzed, and then the conduction mechanism of mature lacustrine shale was addressed. The findings indicate that physical properties have a minor impact on shale electrical properties, while mineral composition (carbonates minerals, clay minerals) and fluid type have a significant impact. Silty-laminated shales (including shell limestone) are affected electrically by carbonate minerals, clay minerals, oil saturation, free water and bound water, argillaceous-laminated felsic shales and argillaceous-laminated clay shales are affected electrically by bound water, adsorbed oil, and clay minerals. For mature lacustrine shale, there are three different types of conductivity mechanisms: clay additional conduction (type Ⅰ), organic matter and clay complex conduction (type Ⅱ), and porous conduction of brittle mineral matrix (type Ⅲ). Silty-laminated shales mainly develop conductivity mechanisms of porous conduction of brittle mineral matrix and clay additional conduction, whereas argillaceous-laminated felsic shales and argillaceous-laminated clay shale primarily develop conductivity mechanisms of clay additional conduction and clay additional conduction. The findings provide guidance for enriching the electrical conductivity mechanism of shale and raising the accuracy of shale saturation interpretation.