The invention provides a constant-volume gas-injection well gas injection quantity design method. The method includes the following steps that: the volume of remaining oil in a gas injection well is calculated; stratum energy needed to be supplemented for the gas injection well is determined; and gas injection quantity needed to be supplemented for the gas injection well is determined. With the constant-volume gas-injection well gas injection quantity design method adopted, a theoretical basis can be provided for constant-volume reservoir gas injection quantity; the utilization ratio of the gas injection quantity of the oil well can be improved; gas injection quantity design can be more specific and scientific; and technical support can be provided for the successful implementation of tertiary oil recovery technologies of oil well gas injection.