An adjustable helmet and a head massager are provided. The adjustable helmet comprises a ring-shaped latitudinal shell and a longitudinal shell connected across the latitudinal shell, at least one of the longitudinal shell and the latitudinal shell has an adjustable means, the inside of the adjustable means has a bidirectional locking structure. In the embodiments, the adjustable means has a bidirectional locking structure which can lock the latitudinal shell or longitudinal shell in bi-direction. When the latitudinal shell or longitudinal shell is disengaged or contracted by the adjustable means and is locked by the bidirectional locking structure, the latitudinal shell or longitudinal shell is fixed in the expanding direction or contracting direction and can't continue to be expanded or contracted inwards. Advantageously, the helmet in use has a constant size, which is useful for users, when the above-mentioned adjustable helmet is applied to a head massager, it's convenient for massage.