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The quantitative interpretation of remaining oil saturation has a great significance for improving oil production and guiding the follow-up development of oil fields. The characteristic gamma rays are recorded to determine the remaining oil saturation, which is from the interaction of high energy neutron emitted by the pulsed neutron logging tool and the element of carbon and oxygen. The interpretation results of the remaining oil saturation are affected by the mineral composition of rock and clay types. In order to understand the influence of lithology on the inelastic gamma ratio of carbon to oxygen and the capture gamma ratio of silicon and calcium, the MCNP was employed to obtain the gamma energy spectrum response of formation with different lithology, porosity and saturation. The results show that when the difference of C/O is calculated by the traditional interpretation model, the influence of lithology on C/O difference results in the interpretation error of remaining oil saturation. So an improved interpretation model was proposed to enhance the precision of saturation by introducing the lithological effects. Meanwhile, the results of the simulation example and field example show that the improved C/O interpretation model guarantees the saturation interpretation accuracy requirements of the sandstone formation and the error of calculation saturation is less than 10% in the mixed formation.