The invention provides a combination material additive manufacturing method of a large-size thin-walled open complex structure. The combination material additive manufacturing method comprises the steps that the machining allowance of 1-2 mm is designed in the width direction of a component of a selective laser melting forming part, and the machining allowance of 2-4 mm is designed on a single edge of a component of a laser melting deposition forming part; a transition area is formed between the laser melting deposition forming part and the selective laser melting forming part; after net forming achieving through a selective laser melting technique is completed, the components and a base material are subjected to distressing annealing treatment; separation between the components and the base material is achieved through linear cutting or sawing machine cutting; continuous transition of structures near a bonding interface is achieved by adopting a two-channel lap forming strategy, smalllight spots, the small powder conveying amount and low energy input parameters, and then efficiency forming of remaining parts is achieved by adopting large light spots, the large powder conveying amount and high energy input; and finally, the whole structure is subjected to solution aging heat treatment. The combination material additive manufacturing method of the large-size thin-walled open complex structure can effectively avoid many problems and realizes efficient and high-precision manufacturing of large-size complex thin-walled components.