A reinforcing device of a roof cornice and drainage ditch one-off pouring formwork comprises two symmetrical vertically fixed type threaded rod assemblies, a top keel transversely arranged between theupper portions of the two fixed type threaded rods, a beam bottom adjustable keel transversely arranged between the lower portions of the two fixed type threaded rods, vertical adjustable side keelswhich are symmetrically arranged on the two sides of the beam bottom adjustable keel and located outside the fixed type threaded rods and two adjustable supporting rods which are symmetrically connected between the midpoint of the beam bottom adjustable keel and the lower portions of the two vertical adjustable side keels on the two sides in an inclined manner. A lower side keel section of each vertical adjustable side keel is perpendicularly connected with the beam bottom adjustable keel through a rotary buckle. A lower inclined keel section of each adjustable supporting rod is connected withthe corresponding lower side keel section through a bayonet. An upper inclined keel section of each adjustable supporting rod is connected with the middle connection component of the beam bottom adjustable keel through a bayonet. The reinforcing device of the roof cornice and drainage ditch one-off pouring formwork is convenient to manufacture, easy to install, firm, convenient to assemble and disassemble and capable of accurately controlling elevation, adjusting the size of ditches and improving construction quality.