Accurate density data and a predictive equation of state (EoS) for the CO2-CH4 system are significantly important for the application of natural gas purification and carbon capture, utilization, and storage (CCUS). In this work, densities of the CO2-CH4 binary system with CO, mole fractions ranging from 0.0998 to 0.8988 were measured at temperatures of 313-353 K and pressures of 3-18 MPa using a magnetic suspension balance, and the compressibility factors and the relative combined standard uncertainties were also calculated correspondingly. The PC-SAFT EoS was employed for the density modeling, and the PR EoS was used for comparison. The measured density data in this work and experimental density data from the literature were used to optimize the binary interaction parameter kg in the PC-SAFT EoS and PR EoS. The predicting abilities of PC-SAFT and PR EoS with the optimized kg were compared to the case without binary interaction (k(ij) = 0). The PC-SAFT EoS with the optimized ku had better performance than the PR EoS with optimized k(ij), and it could predict the density properties of the CO2-CH4 system accurately, with an average absolute deviation (AAD) of 1.16%. Modelings from PC-SAFT with and without the optimized were then compared with the measured densities obtained in this work. The relative deviations for the calculation from PC-SAFT with the optimized kg were within the range from -3.93 to 2.94%. Moreover, the CO2-CH4 binary systems with CO2 mole fractions of 0.7985 and 0.8988 in this work can be regarded as CO2-rich fluids, and their density reductions compared with pure CO2 were evaluated for the measured density data and the values calculated from the modified PC-SAFT.