The present invention relates to the technical field of massagers and provides a head massager and a helmet size adjusting structure thereof. The helmet size adjusting structure includes two racks and an adjusting apparatus, the adjusting apparatus includes a revolving shaft and a gear, an upper part and a lower part of the gear are separately engaged with the racks, and a knob component and a locking component are disposed on the revolving shaft and the structure further includes two tension springs, where front ends of the tension springs are fastened on a rear housing and rear ends of the tension springs are separately fastened on the two racks the knob component includes a knob outer ring, a knob shell, and a gear stand the locking component includes a knob lock, an upper cover of the knob lock, and an elastic element the gear stand is located inside the knob lock, a unidirectional gear ring is disposed on the gear stand, a lock that can be clamped on the unidirectional gear ring is disposed on the knob lock, and the elastic element is disposed on the upper cover of the knob lock and teeth are further disposed on the gear stand, where the teeth are engaged with the gear. In the present invention, a knob and a button are designed as a whole and helmet adjustment is implemented by using two manners, its operation is more convenient, and an inner structure is more reliable.
