The invention relates to a composite building formwork system comprising standard formworks, special-shaped formworks, butt joint clamps, formwork clamps, a connecting insertion plate and key keels. The standard formworks are in butt joint transversely through secondary teeth, primary teeth, secondary fasteners and primary fasteners. The connecting insertion plate is inserted into the joint of the upper transverse formwork and the lower transverse formwork in a row. The butt joint clamps are supported between the two opposite formworks, and accordingly the thickness of a wall can be ensured to reach the standard size. The formwork clamps are clamped at the joints of the primary teeth, the secondary teeth, the primary fasteners and the secondary fasteners of the two frameworks. The key keels are supported on the outer side of a connected formwork module. According to the seamless building formwork joint system, the two formworks are lapped in a flat seam and staggered manner and in butt joint through the secondary teeth, the primary teeth, the secondary fasteners and the primary fasteners to enable contact seams to be tightly jointed and the two formworks to be lapped in a planar manner, so that mud leakage is prevented, and further the lapped seams are tight to enable a building side to be seamless. An insertion connection mode of the special connecting insertion plate is adopted for vertical connection of the formworks; since each formwork is provided with a clamp fixing groove, the special connecting insertion plate can be connected with the vertically connected formworks in an inserted manner flexibly, firmly and reliably, and the seams are tight. Meanwhile, the problems of variable size and deformation in the design scheme can be flexibly solved. The special-shaped formworks are used for solving the problem of internal-external corners and other different design requirement schemes.