Accurate prediction for the liquefaction depth of a porous seabed is crucial to the design of shallow foundations; nevertheless most previous studies are predominantly limited to wave-only conditions. In this study, the combined wave-current induced instantaneous liquefaction of a sandy seabed is investigated analytically. The explicit expression of liquefaction depth under combined wave-current loading condition is derived, which can converge to that under the linear wave condition when the current velocity approaches zero. Parametric study indicates that the effects of imposing a current onto progressive waves on the distribution of excess pore pressures and the corresponding liquefaction depth are unneglectable, especially for the opposite current conditions. Copyright © 2019 ASME.