Disclosed is a method for preparing a corrugated sandwich panel including the steps of: extruding melted polyethylene resin by use of an extruder and a slit nozzle; molding the extruded polyethylene resin to a vertically corrugated inner sheet by use of a plurality of molding rolls; supplying a lower sheet from the lower portion of the inner sheet and an upper sheet from the upper portion thereof; forming a plurality of melting grooves by use of heaters each provided between the upper sheet and the inner sheet and between the lower sheet and the inner sheet such that the melting and bonding between the upper sheet and the inner sheet and between the lower sheet and the inner sheet is conducted; and pressing the upper sheet, the inner sheet and the lower sheet each having the plurality of melting grooves formed thereon by use of a plurality of press rolls.