The invention discloses a building formwork joint filling bar. According to the building formwork joint filling bar, equal-length transverse sides and equal-length vertical sides made of same material sequentially form a long-bar component with a right-angle-shaped groove. The cross section of each transverse side is in the shape of an arch, the outward face is an a circular arc camber, and the face matched with a building formwork is a plane; each vertical side is a equal-thickness thin plate, the vertical sides are perpendicularly connected with the transverse sides to form the joint filling bar with the I-shaped, T-shaped, rightward-U-shaped, L-shaped or h-shaped cross section, the width of each transverse side is 7-20mm, the thickness of the middle of each circular arc camber is 1-2mm, the thickness of each vertical side is 0.6-1.5mm, and the height of each vertical side is 10-18mm. According to the building formwork joint filling bar, the width of each transverse side is far larger than that of a telescopic joint reserved when the building formwork is laid, shielding is tight, anti-leakage effect is good, flow-side residue is avoided in the formwork detaching process after the pouring, manual chiseling is avoided, and the pouring quality and the construction progress are also greatly improved. The building formwork joint filling bar is simple in structure, easy to manufacture, low in matching cost and suitable for being matched with various formworks.