The invention relates to a subway foundation pit large-span support system and a construction method thereof. The system comprises horizontal truss support units horizontally supported between underground diaphragm walls of a foundation bit. The horizontal truss support units are uniformly distributed in parallel and at an interval along length and height directions; and each horizontal truss support unit comprises an upper-plane truss support, a lower-plane truss support and connecting rods vertically connected between the upper-plane truss support and the lower-plane truss support. The construction method comprises the following steps of during a construction process, installing one horizontal support unit at a designed elevation, then excavating a corresponding soil body in layers, utilizing a guide rail to drop the other horizontal support unit to a designed elevation through a lifting power device, excavating a corresponding soil body in layers again, and repeating the steps until finishing construction of a basement. The subway foundation pit large-span support system and the construction method thereof are suitable for earth excavating construction of the deep foundation pit; in addition, the horizontal support units adopt spatial truss structures so as to be high in structural strength; and the support system is adopted to meet the requirements on stability and deformation control of the subway foundation pit, and has the characteristic of large span at the same time.