The invention discloses a large-span underground space deep foundation pit support system. The large-span underground space deep foundation pit support system comprises a deep foundation pit, multiplesupport units are continuously arranged on the inner wall of the periphery in the deep foundation pit, an I-shaped steel plate pile is formed between two adjacent support units, each support unit comprises upper, lower, left and right prefabricated support parts which are in abut joint and spliced, the prefabricated support parts on the left and right ends are fixedly connected to the I-shaped steel plate piles through bolts, and a whole support wall is formed on the periphery inner wall of the deep foundation pit. Waterproof plain concrete is poured into gaps between the support units and afoundation on the inner wall of the excavated deep foundation pit. The four corners are each provided with one I-shaped steel plate pile. Through the prefabricated support parts, splicing combinationof the periphery of the deep foundation pit in the horizontal and vertical direction is carried out, a combined structure is formed by the deep foundation pit and the I-shaped steel plate piles, arrangement of a transverse support and an even wall of the large-span deep foundation pit can be reduced or removed, and the problem that in the prior art, the work face construction cannot be performed in a large-area manner is solved. The system has the advantages of being reasonable in structure, capable of greatly simplifying construction, low in cost, green, environment-friendly, capable of saving investment, safe, portable and the like.