Abstract: Calcium can bring interpretation difficulties to the remaining oil saturation, and it should be corrected in the practical application. In order to study the influence of formation calcium on interpretation of oil saturation with carbon/oxygen logging, a numerical model is set up using Monte Carlo method to simulate net inelastic and capture spectra measured by the Pulsed Neutron Spectral Tool (PNST) in a typical-size cased well in Daqing oilfield. The model is verified by the comparing the simulated results and the formation parameter simulation wells logging data. Variations of C/O and capture Ca yield under different calcium content, porosity and oil saturation are simulated, a correction method of calcium influence on interpretation of oil saturation is established, and a correction formula is deduced accordingly. The logging interpretation of a logging well in Daqing oilfield is compared with the coring analysis, it can be found that the corrected result has the higher conformity with the coring analysis. It shows that the correction method can eliminate the calcium effects and improve the interpretation accuracy of oil saturation.