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.