A protecting casing for a portable electronic device has a shell and a detachable actuating panel. The actuating panel has a curved clamping sheet crossing over a front edge of the portable electronic device. A lock cylinder is mounted between the shell and the actuating panel. When the lock cylinder is locked, the actuating panel is secured. When the lock cylinder is unlocked, the actuating panel is detachable. Therefore, the actuating panel increases the stability for the protecting casing to be mounted around the portable electronic device. Further, a bolt is connected detachably to the actuating panel and the shell to selectively fasten the protecting casing and the portable electronic device at a fixed position so that the portable electronic device is kept from being stolen.