The invention provides a supporting system for a subway pit and a construction method of the supporting system. The supporting system comprises horizontal supporting platforms horizontally supported in a pit support structure, wherein the horizontal supporting platforms are parallelly and uniformly distributed at intervals in the length and height directions of a pit. The supporting system further comprises guide rails, lifting power units and force transferring mechanisms, wherein the guide rails are parallelly arranged at intervals in the length direction of the pit support structure; the lifting power units are connected between the horizontal supporting platforms and the guide rails; and the force transferring mechanisms are connected between the horizontal supporting platforms and the guide rails. As the lifting power units are arranged at the two ends of the horizontal supporting platforms, on one hand, the dead weights of the horizontal supporting platforms are transferred to the longitudinal guide rails, and on the other hand, the lifting power units and the guide rails are used together to realize integral lifting of the horizontal supporting platforms. Compared with conventional concrete or steel tube internal supports, the supporting system has the advantages of being more efficient, higher in safety factor and more stable in structure, and prevents the pit from potential safety hazards caused by the reason that the digging after supporting principle cannot be followed due to brutal construction of the traditional subway pit.