The invention discloses a combined formwork supporting construction method for a plastic formwork and a multi-layer timber formwork of a concrete external wall. The method comprises the construction processes of external wall steel bar binding, external wall formwork erection, concrete pouring, formwork removal and concrete curing. The process of external wall formwork erection comprises the steps of assembling of the plastic formwork and the multi-layer timber formwork, arranging of setting cement supporting rods on horizontal steel bars of the concrete external wall, reserving of split bolt holes in the plastic formwork and the multi-layer timber formwork, installation of the plastic formwork and the multi-layer timber formwork, installation of inner joists and outer joists, installation of split bolts and battening on the inner side of an upper opening of the plastic formwork. The method has the advantages that multiple defects in the prior art are overcome, the advantages of the plastic formwork and the advantages of the multi-layer timber formwork are given to full play, and combined formwork supporting of the two materials makes certain contribution to environment-friendly construction, so that under the condition that construction quality is guaranteed, construction materials are saved, and construction cost is lowered.