The optical technology based on Mach-Zehnder interferometry was successfully applied to a high-pressure liquid CO_2 and water system to measure CO_2 solution density. Experiments were carried out at a pressure range of from 5.0 to 12.5 MPa, temperatures from 273.25 to 284.15 K, and CO_2 mass fraction in solution up to 0.061. CO_2 solution density data were obtained from two sets of experiments. These data were calculated through the fringe shifts induced by density changes inside of the high-pressure vessel, which were directly recorded during the experiments, and a modified version of Lorentz-Lorenz formulation. The experimental results indicated that the density ratio of CO_2 solution to that of pure water at the same pressure and temperature is monotonically linear with the CO_2 concentration in the solution. The slope of this linear function, calculated by the experimental data fitting, is 0.275.