A method of manufacturing a waffle slab includes providing a bottom mold separately on columns and exposing joint heads of the columns from the bottom mold; providing first sets of beam bars separately on the bottom mold to be parallel along a first direction; and providing second sets of beam bars separately on the bottom mold to be parallel along a second direction, wherein the second sets b of beam bars are configured to intersect with the first sets of beam bars without interference and together they form accommodation spaces for waffle molds.