Abstract: Pulse neutron logging plays an important role in monitoring residual oil in the oil field development. The composition and structure of strings are complex in the offshore oil field, and then the C/O value is different owing to the difference fluid volume in the well, so there are influence on determining residual oil saturation. The model of different pipe string, porosity and formation fluid, etc., was established byusing Monte Carlo method,then the inelastic scattering gamma ray spectra under the condition of different string were simulated, and the responses of the C/O values to the porosity and fluid saturation were studied. At last, we obtained the residual oil saturation interpretation model under different pipe string, and the results were credible by processing data of two wells. The result provides technical support for monitoring remaining oil saturation in offshore oil field.