The parameters of C/O ratio and Sigma based on pulsed neutron source can be obtained at the same time in one logging, which utilizes a special pulse and measurement timing design to record inelastic, capture gamma ray spectra and gamma ray time spectrum at the far and near detector. By combining dual parameters for determining oil, water and gas saturation, it can resolve the problem no way to distinguish gas and water only using C/O, oil and fresh water only using Sigma, in addition, it can determine saturation under the condition of unknown borehole porosity information. By theoretical analysis and Monte Carlo simulation method, the gamma ray time spectrum after repetitive pulse of neutron emission can still meet the exponential decay law, and repetitive pulse before neutron lifetime measuring can raise gamma count rate of time spectrum 3 to 5 times, therefore, the dual parameters measured simultaneously in one logging are feasible. Through dual parameters of C/O ratio and Sigma, the C/O data is cross plotted against Sigma data and then to determine oil, water and gas saturation. By using Monte Carlo method to set up computation model, inelastic gamma spectrum, capture gamma spectrum and time spectrum under certain borehole and formation conditions can be simulated, and the logging response is given when determining oil, water and gas saturation by using cross plot technique of C/O and Sigma. In addition, the influence of lithology, porosity, shale content, borehole size, formation water salinity and gas density on determining saturation were simulated, and it will provide response relation for determining oil saturation by dual parameters of C/O ratio and Sigma.