The low permeability of shale matrices necessitates overcoming a threshold pressure gradient to initiate hydrocarbon flow, which poses a major constraint on recovery efficiency. However, the microscopic mechanisms underlying the threshold pressure gradient, particularly the roles of interfacial interactions and pore confinement, remain unclear. A comprehensive understanding of the threshold pressure gradient is essential for enhancing recovery strategies and improving shale oil extraction efficiency. This study provides a comprehensive analysis of the interfacial and size effects on the threshold pressure gradient within kerogen, quartz, and portlandite pores using molecular dynamics simulations. A method for assessing molecular thermal motion and quantifying the threshold pressure gradient was developed using molecular dynamics simulations. The results indicate that the threshold pressure gradient decreases in the order of kerogen, quartz, and portlandite pores. The adsorption characteristics of shale oil components at the interface were clarified through density distribution and molecular behavior analysis, and the factors contributing to the threshold pressure gradient were identified. It was found that the threshold pressure gradient is significantly influenced by the strength of interfacial interactions between the polar shale oil components and the solid matrix. Additionally, an analytical model was proposed to predict the correlation between the threshold pressure gradient and the pore size, which can extend the prediction of the threshold pressure gradient to a larger scale of thousands of nanometers. These findings offer insights into shale oil recoverability in nanopores and provide theoretical guidance for its extraction.