The invention discloses a process of large ultrasonic seal welding for a defrosting air duct and an instrument panel. The process comprises the steps of 1, loading, 2) compression molding of an instrument panel body, 3) compression molding of a defrosting air duct, 4) preheating, 5) welding of the instrument panel body and the defrosting air duct, 6) cooling, and 7) weld joint treatment. In the process provided by the invention, a PLC computer control system is employed for the welding of the defrosting air duct, allowing safe and accurate motions, and high-precision welding at a high speed and in high efficiency; a built-in protection system is provided to ensure safe and stable welding. In addition, according to a large ultrasonic welding technique adopted in the process provided by the invention, a sudden increase of acoustic resistance at a welding interface causes a local high temperature such that the defrosting air duct is reliably welded to the instrument panel body; the whole procedure is controlled full automatically, and integration of the instrument panel and the defrosting air duct is achieved without glue and bolt. Moreover, the process provided by the present invention performs strict control on the cooling step after welding, thus guaranteeing the forming quality of a welding junction.