The invention provides a fabricated horizontal supporting system for a subway foundation pit and a fabricated construction method of the fabricated horizontal supporting system. The fabricated horizontal supporting system comprises horizontal supporting platforms horizontally supported in a foundation pit support structure, wherein the horizontal supporting platforms are parallelly and uniformly distributed at intervals in the length and height directions of a foundation pit; and force transferring mechanisms are connected between the horizontal supporting platforms and the foundation pit support structure and uniformly distributed at intervals along the side surfaces of the horizontal supporting platforms. The force transferring mechanisms are utilized to realize effective fabricated connection between the horizontal supporting platforms and the foundation pit support structure and are arranged between the horizontal supporting platforms and the foundation pit support structure, so that the force transferring mechanisms can drive the horizontal supporting platforms to vertically move along the foundation pit support structure under the condition of bearing horizontal supporting forces, accordingly, quick vertical movement of multiple layers of the horizontal supporting platforms is facilitated, all-layer adjustment in the fabricating process is guaranteed, the construction efficiency is high, and the fabricated horizontal supporting system is safe and effective.