The invention belongs to the technical field of floor system construction, and relates to a construction method of an XPS body inner die ultra-thick long-span floor system. The construction method comprises the processes of die supporting frame construction, template erection, reinforcing steel bar and XPS block body inner die location, main beam, secondary beam and plate bottom reinforcing steel bar binding, installation engineering pipeline embedding, XPS block body inner die bottom portion anti-floating iron wire binding, XPS block body inner die manufacturing, XPS block body inner die placing, XPS block body inner die top portion anti-floating iron wire binding, panel reinforcing bar installing, concrete pouring and the like. The construction method has the advantages that an XPS block body inner die is capable of bearing the pressure of 1.5 KN / m<2>, anti-leakage and installation project construction is benefited, manufacturing cost is low, construction is convenient, construction progress is fast, and the XPS block body inner die is applicable to buildings with various spans and loads, especially applicable to long-span, large-load, large-space, multiple-demand and high-rise buildings like commercial buildings, supermarkets, multiple-layer parking lots, office building large-space meeting rooms and other public buildings, also applicable to civil buildings like hotels and large-bay residences, and further applicable to multiple-story or single-story industrial factory buildings, storehouses, garages and the like.