The invention relates to a warning board for a construction site. The warning board comprises a triangular frame, a supporting rod and movable supporting legs, wherein the three faces of the lower end of the triangular frame are movably connected with the movable supporting legs through positioning pins; the lower ends of the movable supporting legs are movably connected with adjusting rods through elastic buckles; adjusting holes are evenly formed in the outer surfaces of the lower ends of the movable supporting legs; and the elastic buckles are movably installed inside the adjusting holes. According to the warning board for the construction site, the movable supporting legs can be opened towards the outer side through the positioning pins, the movable supporting legs and the triangular frame make contact with one another through magnet attraction, and therefore the movable supporting legs are more convenient to fold and unfold and can be fixed through magnet attraction after being unfolded, and stability is good; then adjusting blocks movably installed inside cube grooves can be vertically adjusted through the elastic buckles, the height of the warning board can be adjusted conveniently, and the warning board is convenient to store and transport; and meanwhile, the interiors of the adjusting blocks are movably connected with the adjusting rods through threads, fine adjustment of the adjusting rods can be conducted, and stable installation of the movable supporting legs can be facilitated when the ground is uneven.