The disclosure discloses a concealed bicycle lock system and a setup method thereof. The lock system of the disclosure includes a lock with an upper shaft and a lower shaft. The upper shaft is configured to link and move with a handlebar via a stem; the lower shaft is configured to link and move with a front wheel by embedding the lower shaft into a fork tube. When the lock is in a locked mode, the upper shaft and the lower shaft are not linked to move together. In contrast, the upper shaft and the lower shaft are linked to move together in an unlocked mode. A user may set up the lock without performing destructive modification to the bicycle.