Instantaneous liquefaction can be induced by the upward seepage force generated in a sandy seabed under wave troughs. The wave-induced pore-pressure distribution in the non-cohesive seabed with an instantaneously-liquefied layer is investigated analytically. Based on the analytical solution to Biot's consolidation equations for the response of a poro-elastic bed to water waves, the expressions of liquefied soil depth are derived with modifying the criterion for instantaneous liquefaction, which is verified with existing offshore field observations and multi-scale numerical simulations. Analytical investigation indicates that, in the instantaneously-liquefied soil layer, the buoyant weight of soil is essentially balanced by upward seepage force into a quasi-static state. Underneath the fully liquefied layer, the effective stress can be remarkably reduced. An excess pore-pressure ratio is then proposed to quantitatively evaluate the instantaneous liquefaction degree and its corresponding influential depth. Parametric studies indicate that, for a certain excess pore-pressure ratio, the influential depth decreases with increasing saturation degree, permeability of soil or water depth, but increases with increasing wave height. The influential depth of a non-cohesive seabed under waves increases significantly with reducing the threshold value of excess pore-pressure ratio, which may have much influence on the stability design for submarine structures.