The invention relates to a standard steel frame formwork system for concrete floor slabs and a construction method thereof. The system comprises square steel girders, steel frames, saddle girders, vertical bars and cross bars, wherein the cross bars are tied and fixed with the adjacent vertical bars by fasteners; U supports are installed on the vertical bars; the square steel girders are installed on the U supports and serve as main keels; the saddle girders are placed on the main keels; the steel frames are placed on the saddle girders and serve as secondary keels; and formworks are paved onthe steel frames. The construction method comprises the following process flows: snapping floor slab formwork erecting elevation lines; placing cross position lines of the vertical bars; installing vertical bar crossers on the cross position lines; installing the vertical bars; installing the cross bars; installing the U supports; installing the square steel girders; installing the saddle girders; installing the standard steel frames; levelling lines; paving multi-layer slabs fully; adjusting, arching and reinforcing; cleaning; inspecting and accepting; and removing the formwork. The system and the construction method have the following beneficial effects: the system has the characteristics of factory processing and tool-typed installation; and the construction method has the advantages of good stability, safety, reliability, time and labor saving, high construction speed and high turnover rate, can save lots of wood and funds, and meets the requirements for energy saving, environmental protection and economic benefits.