The present invention relates to a head massager and a helmet size adjustment structure therefor. In prior art designs, to adjust a helmet for an existing head massager in terms of size it is generally required that a first housing and a second housing is moved using a pulling action by hand. This kind of structure, which requires manual spacing adjustment, is not easy to operate and has relatively poor reliability. To address these problems, the present invention provides an adjusting apparatus that allows the helmet size to be adjusted through rotating and pressing actions. The helmet size adjustment structure comprises two racks (300) and an adjusting apparatus (400), the adjusting apparatus (400) comprising a rotary shaft (410) and a gear wheel (420), an upper part and a lower part of the gear wheel (420) respectively meshing with the racks (300), and a knob assembly (500) and a locking assembly (600) being provided on the rotary shaft (410), and further comprising two draw springs (700), the front end of each draw spring (700) being fixed onto a rear housing (200), the rear end of each draw spring (700) being respectively fixed onto the two racks (300). The knob assembly (500) comprises a knob outer ring (510), a knob outer shell (520), and a gear wheel seat (530). The locking assembly (600) comprises a knob fastener (610), a knob fastener upper cover (620) and an elastic member (630). The gear wheel seat (530) is located in the knob fastener (610), a one-way toothed ring (531) is provided on the gear wheel seat (530), a fastener (611) capable of clamping on the one-way toothed ring (531) is provided on the knob fastener (610), the elastic member (630) is arranged on the knob fastener upper cover (620), teeth (532) are also provided on the gear wheel seat (530), and the teeth (532) mesh with the gear wheel (420), wherein the knob is designed as one piece with a push-button. The helmet is adjusted by two methods, the operating structure is more convenient and th
