Accurate estimation of CH4 absolute adsorption amount is essential for shale gas-in-place (GIP) evaluation as well as well productivity. Recent studies have shown that pore size distribution (PSD) plays an important role in determination of the absolute adsorption. However, previous studies only contain some discretized pore sizes, while a continuous PSD has not been fully taken into account. In this work, CH4 adsorption behaviors in various nanopores are first investigated via the grand canonical Monte Carlo (GCMC) simulations. The CH4 adsorption in nanopores is divided into six distinct adsorption types based on density distributions. Then, the Ono-Kondo (OK) model with PSD lumping is used to characterize CH4 absolute adsorption in kerogen nanoporous media with pore sizes ranging from 0.7 to 50 nm. The validity of our proposed OK model with PSD lumping is tested by 5 cases with varying micropore volume proportions from 5 to 35%, with each case containing 250 sets of randomly generated PSD samples. We find that by fitting the excess adsorption isotherm, the OK model with PSD lumping has an excellent agreement in terms of the absolute adsorption amounts with those obtained from the GCMC simulation, while deviations increase as micropore volume proportion increases. Overall, the OK model with PSD lumping outperforms the popular single-layered Langmuir and SDR models as well as multilayer models such as supercritical BET (SBET) and single-parameter OK model without PSD considerations for absolute adsorption predictions in kerogen nanoporous media with a continuous PSD.