In this paper, thermodynamic phase behaviour and miscibility of confined pure and mixing fluids in nanopores are studied. First, a semi-analytical equation of state (EOS) is developed, based on which two correlations are modified to predict the shifts of critical temperature and pressure. Second, the thermodynamic free energy of mixing and solubility parameter are derived, quantitatively calculated, and applied to study the conditions and characteristics of the fluid miscibility in nanopores. Third, an improved EOS model with the modified correlations is proposed and used to calculate the phase properties and miscibility-associated quantities of three mixing fluids. The critical temperature and pressure of confined fluids are always decreased by reducing the pore radius. The negative pressure state is validated for a confined liquid, whose upper temperature limit is quantitatively determined and found to be lowered with the reduction of pore radius. The liquid-gas miscibility is beneficial from the pore radius reduction and the intermediate hydrocarbons (e.g., C-2, C-3, i- and n-C-4) perform more miscible with the liquid C-8 in comparison with the lean gas (e.g., N-2 and CH4). Moreover, the molecular diameter of single liquid molecule is determined to be the bottom limit, the pore radius above which is concluded as a necessary condition for the liquid-gas miscibility. The calculated phase behaviour and minimum miscibility pressures (MMPs) of the three mixing fluids agree well with the literature results, which reveals that the shifts of critical properties dominate the phase behaviour and miscibility changes of confined fluids from bulk phase to nanopores.