A helmet shield opening mechanism is provided, wherein first and second constraint tracks are provided on at least one support leg of a shield jaw guard, and a driver element is configured to come into contact with and drive the two tracks, so that the helmet has forced and controlled modes relative to the opening action and process of the helmet body. On one hand, the operational reliability and flexibility of the shield opening mechanism can be improved; on the other hand, the shield is opened smoothly without collision, thus improving the wearing comfort of the helmet. Further, a helmet with changeable jaw guard equipped with the helmet shield opening mechanism can plan the opening process of the shield flexibly to adapt to states of the jaw guard at different operation positions, thereby greatly reducing the chances of collision between the jaw guard and the shield.