The salinity of the mixed water in the water flooded zones injected by the fresh water and the sewage has a great variation. As an important parameter in a remaining oil saturation calculation model in a cased hole, the macroscopic capture cross section of the mixed formation water is difficult to determine. The distribution of the salinity and solutes in the mixed formation water from the water flooded layer in the study area shows that it is necessary to calculate the macroscopic capture cross section of the mixed formation water layer by layer. Pulsed neutron-neutron well logging evaluation in the water-flooded zone is taken as an example in this paper. Firstly, the original oil saturation model is established. And then the macroscopic capture cross section of the formation fluid is determined. Finally, the macroscopic capture cross section of the mixed formation water is calculated based on the static and dynamic logging information. As the water flooding degree of some sampling points is inhomogeneous in the same layer, the method can also calculate the corresponding macroscopic capture cross section of the mixed formation water. The practical application shows that this method accurately reflects the variability of the salinity of the formation water. The calculation accuracy is improved because the difference between layers has been taken into consideration. This method also provides a new way for the quantitative assessment of the water flooded zones injected by the fresh water and the sewage by using some comprehensive methods to consider the dynamic and static logging data.