An adjustable neck brace worn at a human neck for fixing a cervical vertebra includes a cover panel and an adjusting device fixed onto a surface of the cover panel, and the adjusting device includes an adjusting rod, a limit member and an adjusting knob disposed on a surface corresponding to the adjusting rod, and an end of the limit member includes a bracket, and the adjusting knob links an adjusting gear installed at the adjusting rod, and an end of the adjusting rod includes a height adjusting plate having a supporting portion for supporting a chin. With the foregoing technical measures, the invention features a secured and stable structure to maximize the effects of medical treatment and physical therapy. The height of the supporting portion can be adjusted to provide users a feedback about a definite feel of the adjustment through the bracket.