Micro-nano pores are developed in unconventional oil and gas reservoirs such as shale and tight sandstone, and the study of oil-water two-phase seepage law and relative permeability in micro-nano pores is the theoretical basis for the effective development of such reservoirs. Addressing the two-phase flow of oil and water within nanopores, a mathematical model was established for nanoporescale two-phase oil-water flow, grounded on the Hagen-Poiseuille (HP) equation and taking into account microscale seepage mechanisms such as oil-water distribution, viscous coupling, multi-layer adsorption, and slippage. Based on this model, a calculation method for relative permeability was developed. The validity of the model was verified by fitting the results from Lattice Boltzmann method(LBM) simulations. Furthermore, through a parametric sensitivity analysis, the characteristics of two-phase oil-water flow and the influence patterns of relative permeability within nanopores were investigated. The results showed that the slip length significantly impacted the velocity distribution of oil and water. An increase in the wetting contact angle led to varying degrees of augmentation in both oil and water relative permeabilities. As the viscosity ratio rose, the oil-phase relative permeability experienced a notable increase, while the water-phase relative permeability remained relatively unchanged. The presence of positive slip could result in relative permeabilities exceeding unity. As the pore radius enlarged, the pore area available for fluid flow expanded, thereby enhancing both oil and water relative permeabilities. This study holds guiding significance for elucidating the fluid flow mechanisms in micro-porous media and facilitating the exploitation of shale oil.