Top-down decreasing-formwork formwork system and construction method

The invention discloses a top-down decreasing-formwork formwork system and a construction method thereof, wherein the decreasing-formwork formwork system comprises a main beam, a secondary beam and a formwork fixed on the secondary beam; and the formwork is pre-provided with a column hole for being passed through by a structural column at the position of the structural column. The formwork system is characterized by also comprising a hanger rod and a chain hoist; the hanger rod is of a matrix structure and vertically arranged on the whole decreasing-formwork formwork system; the lower end of the hanger rod is fixedly connected to the main beam; the length of the hanger rod is larger than the floor height of a basement structure; the hanger rod is provided with a lifting nut; and the suspension point of the chain hoist is arranged at the hanger rod position of the surrounding of a post cap. The formwork system takes the firstly completed structure as the reacting force point of the lower structure construction, and falls off layer by layer along the construction process, thus being capable of effectively reducing the overexcavation depth, canceling the cushion coat, avoiding the waiting time for concrete curing, being very beneficial to the control on the surrounding environment, and speeding up the top-down construction.