At present, commercial oil flow has become one of the important economic evaluation indexes of the initial reserve calculation standard. The Chinese petroleum industry standard(Q/SY 01180—2020) Guideline for evaluation of petroleum economical ultimate recovery specifies the calculation method for the commercial oil flow of a single well with stable decline. However, it does not cover the calculation method for single wells experiencing a “two-stage” decline, characterized by a rapid decrease in production at the initial stage and a relatively gentle decline in the middle and late stages. In this regard, based on the single well commercial oil flow calculation method of single decline rate, this paper establishes a single well commercial oil flow calculation model that aligns with the oil “two-stage” decline pattern by applying the principle of profit and loss balance and incorporating the oil “two-stage” decline pattern. Taking the Sun Formation in the Liaohe Depression as a case study and considering factors such as drilling investment, operation cost, oil and gas price, the model is applied to determine the single well commercial oil flow under the evaluated oil price of 60 USD/barrel. The research results provide reference for reservoirs of the same nature.