Abstract: The widespread distribution of nanoscale pores in shale makes the phase behavior of alkenes different from that of conventional reservoirs. The differences in surface properties of different minerals will have different effects on fluid phase behavior. Monte Carlo molecular simulation method was used to simulate the gas-liquid phase equilibrium of pure alkanes in the pores of kerogen and calcite. The phase equilibrium parameters and critical parameters were calculated. The mechanism of micro-scale effects on the gas-liquid phase equilibrium and critical parameters was analyzed by density distribution. The phase behavior of the fluid in the shale reservoir is affected by adsorption and capillary condensation, which leads to a higher gas-phase density and a lower liquid-phase density. Under the influence of equilibrium phase, the critical temperature and pressure of alkanes in nanopores are lower than those in bulk phase. With the increase of pore size, the properties and critical parameters of vapor-liquid equilibrium gradually approach the bulk value. The change law of phase behavior of alkanes in shale organic pores is expounded and the critical parameters are quantitatively characterized, which provides support for accurate calculation of phase state in shale organic pores.