Based on the analysis of the production composition of reservoirs developed by the second & tertiary recovery combination (STRC), the relationship between the overall output of the STRC project and the production level during the blank water flooding stage is proposed. According to the basic principle of reservoir engineering that the “recovery factor is equal to sweeping coefficient multiplied by oil displacement efficiency”, the formula for calculating the ultimate oil recovery factor of chemical combination flooding reservoir was established. By dividing the reservoir into a series of grids according to differential calculus thinking, the relationship between the ultimate recovery factor of a certain number of grids and the recovery degree of the reservoir was established, and then the variation law of oil production rate of the STRC reservoir was obtained. The concept of “oil rate enlargement factor of chemical combination flooding” was defined, and a production calculation method of reservoir developed by STRC was put forward based on practical oilfield development experience. The study shows that the oil production enhancing effect of STRC increases evenly with the increase of the ratio of STRC displacement efficiency to water displacement efficiency, and increases rapidly with the increase of the ratio of recovery degree at flooding mode conversion to the water displacement efficiency. STRC is more effective in increasing oil production of reservoir with high recovery degree. Through practical tests of the alkali free binary flooding (polymer/surfactant) projects, the relative error of the oil production calculation method of STRC reservoir is about ±10%, which meets the requirements of reservoir engineering.