The invention discloses a thin-wall matrix for filling cast-in-site reinforced concrete and a manufacturing method thereof. The integrated matrix bottom plate and the ambient side wall are made of the mixture from hydration reaction, such as wet cement mortar and light cement mortar; then an inner mold support plate is hoisted and supported in or on an opening surrounded by the ambient side wall; then the top surface of the inner mold support plate is coated with the mixture from hydration reaction such as wet cement mortar and light cement mortar to form a top plate of the thin-wall matrix; when in forming, the periphery of the top plate and the corresponding wet ambient side wall are integrated into a whole body; the whole body is then let stand and maintained so that the bottom plate, the ambient side wall and the top plate are cured synchronously into a whole. The timing of the method for coating the cement mortar of top surface is to choose the wet condition where the ambient side wall has the substantially same physical and chemical properties as the top surface cement mortar. The method can achieve the integrated structure formed by the synchronous curing of the ambient side wall and the bottom plate, and overcomes the shortcomings with the prior manufacturing method.