The invention provides an internal mold fixing and external mold assembling type disassembly-free composite formwork and a mounting method, and belongs to the field of building materials. The composite formwork comprises an inner formwork, a heat preservation layer, a fireproof layer, a panel layer, an outer decorative surface decorative layer, secondary keels, main keels and diagonal braces; theheat preservation layer, the fireproof layer, the panel layer and the outer surface decorative layer are compounded; embedded screws are arranged in the heat preservation layer; the embedded screws are sequentially connected with the inner formwork, the secondary keels and the main keels through connecting pieces and bolt assemblies; the secondary keels are perpendicular to the main keels; the bolt assemblies are connected with the diagonal braces; and the diagonal braces are fixed to the base. The mounting method comprises the following steps: S1, the inner formwork is fixed through the secondary keels, the main keels and the diagonal braces; S2, the heat preservation layer, the fireproof layer, the panel layer and the outer surface decorative layer are assembled into an outer formwork; S3, the outer formwork is fixed to the inner formwork through the connecting pieces; and S4, steel bars are arranged between the inner formwork and the outer formwork, and concrete is poured. Comparedwith the prior art, modular production is adopted, all parts are in factory operation, the site construction efficiency is improved, the construction period is shortened, the labor cost is reduced, environmental pollution caused by site construction is reduced, and firmness and safety are higher.