The invention discloses a manufacturing device for a big insulating cardboard with high density as well as a manufacturing method thereof. The device comprises a pulp feeding system, a pulp purifying and adjusting system, an adhesive paste paper pulp forming system and an insulating cardboard forming system. The pulp feeding system comprises a hydraulic pulp kneader, a pulp storage tank, a slag separator, a double-disc pulp grinder and a pulp forming tank which are connected sequentially through pipes. The pulp purifying and adjusting system comprises a pulp adjusting box, a desilting disc, asand separator and a pulp separating box which are connected sequentially through pipes. The adhesive paste paper pulp forming system comprises a round net pulp spraying former, an adhesive paste paper pulp forming device and a delivering machine. The insulating cardboard forming system comprises a pre-loading machine, a board loading machine, a hot pressing machine, a board unloading machine anda board receiving table. The invention adopts pure wood pulp as a raw material, and a set of advanced devices to manufacture big insulating cardboards with high density, and a gap in the insulating cardboard manufacturing field is filled.