The invention discloses a preparation method of composite coating on the inner wall of a reduction furnace bell jar. The preparation method comprises the following steps that a transition layer is prepared, and specially, a transition layer is prepared on the inner wall of the stainless steel bell jar by adopting thermal spraying; raw materials of the transition layer are an alloy consisting of one or more of aluminum, iron, copper, nickel, chromium, zinc, vanadium, manganese, silicon, cobalt, silver, cadmium, zirconium and tungsten; an anti-fouling coating with high density, high bonding strength, high thermal stability and low defect rate are prepared by using gas power spraying on the surface of the transition layer; and the raw materials of the anti-fouling coating are an alloy consisting of one or more of silver, gold and platinum. According to the preparation method, on one hand, the process steps can be simplified, the production cost can be reduced, the problem of the defect ofweld joints of a spliced plate can be solved, on the other hand, the structural stability of the coating can be improved, and the service life of the coating can be prolonged.