The invention belongs to the technical field of helmet, relating to a variable jaw structure protecting helmet. The helmet comprises a helmet main body, a jaw guard and two mould boards; wherein the jaw guard is provided with two fork handles, the two mould boards are respectively arranged at the two sides of the helmet case main body, the mould board is fastened and connected with the helmet case main body or the mould boards are connected with the helmet main body into a whole, each mould board is opened with two groove-shaped guide rails and is additionally provided with four movable pins, one end of each movable pin is inserted into the four guide rails on the two mould boards, each fork handle is respectively connected with the two movable pins, the jaw guard drives the movable pin by virtue of the fork handle, each movable pin moves along respective corresponding guide rail, and the position of the jaw guard can be changed under the constraints of the movable pin and the guide rail. Compared with the prior art, the invention has the advantages that no guide rail is arranged on the jaw guard, bending strength of fork handle is improved, fracture resistance of the fork handle of the jaw guard is improved, besides, stress concentration hidden danger can be effectively eliminated by abandoning the chute of the fork handle, and meanwhile strength of jaw guard is improved.