The helmet comprises a helmet body, a backward turning neck
protection mechanism, a rotation adjusting mechanism and an adjusting auxiliary mechanism, the backward turning neck
protection mechanism comprises a rotation plate, an inner fixing
pipe, a rotation shaft and a backward turning protection frame, and the rotation adjusting mechanism comprises a clamping groove, an adjusting
pipe, a rotation frame, a telescopic frame, a rotation pushing block and a rotation frame. The adjusting auxiliary mechanism comprises a rotating
pipe, a movable rod, a tightening block, a tightening spring, a matching block and a rotating groove, the backward turning neck
protection mechanism can cover and protect the neck area of a wearer, and the risk of
neck injury can be remarkably reduced when an accident occurs; the turning tightness of the backward turning protection frame can be adjusted step by step to adapt to wearers with different head circumferences, the adjustment process is smoother due to the design of the rotating pipe and the deep groove hole, and the size of the helmet can be conveniently and rapidly adjusted.