A locking device adapted to lock a shell to an installation body includes a thread connection member extending through a connecting hole in the shell and engaging a threaded hole in the installation body, a sealing ring sleeved on the thread connection member and adapted to be pressed on an installation surface of the shell to seal the connecting hole of the shell, and a gasket sleeved on the thread connection member and adapted to be pressed between the thread connection member and the sealing ring to prevent the thread connection member from being loosened. The gasket has a pair of stopping arms. An angle defined between the stopping arms is greater than 0 degrees and less than 180 degrees. A pair of top ends of the stopping arms are adapted to simultaneously abut against an outer wall of the shell to prevent the gasket from rotating with the thread connection member.