The invention discloses a determination method of a well killing occasion of a pressing-back method for a gas well. The determination method of the well killing occasion of the pressing-back method for the gas well comprises the following steps of collecting the physical property parameters of a stratigraphic rock and a fluid through geological logging and the data of an adjoining well; drawing an IPR (Inflow Performance Relationship) curve of the gas well by a formula through the collected data; drawing an OPR (Outflow Performance Relationship) curve of a relationship between a pit shaft gas yield and bottom hole pressure; drawing the IPR curve and the OPR curve in the same coordinate system; calculating an allowable maximum well shut-in time; calculating a wellhead pressure function when the well killing is started at some moment after well shut-in; solving wellhead maximum pressure; judging latest time when the well killing is started. The method is applicable to the conditions that the circulating well killing cannot be carried out due to the well blowout of the gas well and the pressing-back method is adopted to carry out the well killing; the latest well killing time that the well is successfully killed by using the pressing-back method after the well shut-in can be calculated; therefore, the risk that wellhead pressure generated in a well killing process exceeds maximum allowable wellhead pressure can be removed; the theoretical guidance is provided for the success of the well killing. The method is uncomplicated in calculation process, simpler in operation technique, and suitable for field use.