An underfill encapsulant for a wafer packaging and its application method are provided to cut clearly a wafer into plural chips by using by directly applying the underfill encapsulant before transferring a semiconductor wafer to the chips. An underfill encapsulant for a B-stage process contains a thermosetting resin, an imidazole phosphate salt catalyst, at least one resolvent, at least one fluxing agent, and a selective component, which is selected from the group consisting of a surfactant, a moisturizer, an antifoaming agent, a coupling agent, an inorganic condensing agent, an adhesion promoter, a flow additive, an air release agent, and a combination thereof. The encapsulant is cured at a first temperature during the B-stage process and provides a planar and non-viscous surface on a semiconductor wafer or a silicon chip. The overall encapsulant is cured at a second temperature, which is higher than the first temperature.