This work applied molecular dynamics (MD) simulation to calculate densities of natural gas mixtures at extremely high pressure (>138 MPa) and high temperature (>200 ℃) conditions (xHPHT) to bridge the knowledge and technical gaps between experiments and classical theories. The experimental data are scarce at these conditions which are also out of assumptions for classical predictive correlations, such as the Dranchuk & Abou-Kassem (DAK) equation of state (EOS). Force fields of natural gas components were carefully chosen from literatures and the simulation results are validated with experimental data. The largest relative error is 2.67％for pure hydrocarbons, 2.99％for C1/C3 mixture, 7.85％for C1/C4 mix-ture, and 8.47％ for pure H2S. These satisfactory predictions demonstrate that the MD simulation approach is reliable to predict natural- and acid-gases thermodynamic properties. The validated model is further used to generate data for the study of the EOS with pressure up to 276 MPa and temperature up to 573 K. Our results also reveal that the Dranchuk&Abou-Kassem (DAK) EOS is capable of predicting natural gas compressibility to a satisfactory accuracy at xHPHT conditions, which extends the confidence range of the DAK EOS.