The invention discloses a manufacturing process for an ultra light fiber board. The method comprises the following steps of: peeling wood, cutting to obtain sheets, screening, washing, boiling, separating fibers, mixing and applying adhesive, drying and sorting, paving and forming, prepressing, performing hot-pressing, turning the board and cooling, sanding, inspecting and classifying, and packaging and putting in storage, wherein in the boiling step, the boiling material level is 1.6 to 3m, and the boiling temperature is 165 to 175 DEG C; in the step of separating the fibers, the wood sheets enter a mill through a ribbon spiral conveyor, molten waxes are fed into the mill simultaneously, the application amount is 5.0 to 7.0kg/m<3>, and the gap of an abrasive wheel is 0 to 4mm; and in the step of mixing and applying the adhesive, melamine modified urea resin adhesive is applied, the adhesive application amount is 195 to 210kg/m<3>, the adding amount of a curing agent is 1 to 2.5 percent based on the mass of absolute drying adhesive, and the curing agent is a mixture of ammonium chloride and a (commercially available) HA101 cross-linking agent in a mass ratio of 3:1. Moreover, a method for preparing the melamine modified urea resin adhesive used by manufacturing the ultra light fiber board is correspondingly provided for matching the manufacturing process.