Shallow gas is considered one of the most serious geological hazards in deepwater drilling because it has the characteristics of suddenness and is difficult to deal with. To perform a quantitative evaluation of shallow gas risk during deepwater drilling, a numerical model for calculating gas invasion volume is established based on gas-water two-phase flow theory. The model considers the effect of the dynamic drilling process, and the influencing factors which affect the gas invasion volume are analyzed. Results indicate that the gas invasion rate and accumulated gas invasion volume increase with increasing bottom-hole pressure difference. A linear relationship exists between gas invasion volume and bottom-hole pressure difference. The duration of gas invasion increases as the shallow gas zone thickness increases, and the accumulated gas invasion volume grows as shallow gas zone thickness increases. The increase in formation permeability, water depth, and rate of penetration will enhance the gas invasion rate. However, these three factors can hardly affect the accumulated gas invasion volume. The gas flow rate increases significantly with increasing burial depth of shallow gas. On the basis of influencing factor analysis, a series of methods that consider different risk levels is proposed to control shallow gas, which can provide a reference for the prevention of shallow gas disasters during deepwater drilling.