The invention relates to a construction method of a ventilating duct outlet of a prefabricated type concrete roof. A typified formwork is manufactured and assembled and poured in a workshop, a prefabricated ventilating duct outlet is manufactured, and holes are evenly formed in the bottom of the prefabricated ventilating duct outlet; the prefabricated ventilating duct outlet is transported into ayard; a control line is mounted on a reserved hole of a roof ventilating duct in a popup mode, rebars are embedded into the periphery of the reserved hole of the ventilating duct of a cast-in-place roof plate, and then a grouting material on the cast-in-place roof plate is leveled; the prefabricated ventilating duct outlet is hoisted in place through hoisting equipment, the rebars embedded into the roof plate are correspondingly inserted into the holes in the bottom of the prefabricated ventilating duct outlet, grouting is carried out at the positions of connecting gaps in order to connect theroof panel with the prefabricated ventilating duct outlet, and the embedded rebars and a body are reliably fixed. The stability is high, the sealing performance is good, the anti-permeability performance is good, and the perception is excellent. The purposes that the construction progress is accelerated, the cost is reduced, the efficiency is improved, the quality is guaranteed, and the occurrence frequency of site safety accidents is reduced are achieved.