A bracket device is configured to mount a slide rail to a post. The bracket device includes a bracket, a hook, a locking element and an extension element. The bracket has a first engagement part. The hook is connected to the bracket and located at a close position relative to the bracket. The locking element is operatively engaged with the first engagement part of the bracket for abutting against the hook to hold the hook at the close position. The extension element is connected to the locking element. Wherein, when the locking element is operatively disengaged from the bracket, the locking element no longer abuts against the hook, and the hook is no longer held at the close position relative to the bracket.