The present invention discloses an improved casing assembly for burglar alarm, and the casing comprises an upper casing, a chassis and a front panel; wherein a circuit board and a speaker are included in the upper casing and the chassis and the front panel is covered on the exterior of the upper casing and locked with the chassis by a lock as a whole. The assembly of the forgoing components is connected by the corresponding coupling components on the assembly and thus does not require additional coupling device, which makes the assembly much simpler and quicker and has the advantages of lowering the manufacturing and labor costs. In the meantime, the printed circuit according to the present invention is coupled to a vibration switch or magnetic spring switch as a sensor triggering source for acting as a trigger of the burglar alarm for motor vehicles, portable objects, displayed items, doors and windows, security or display windows. All these burglar alarms can share the same casing and thus can save the cost for manufacturing the molds. The casing can go with a rope having a metal head for passing through, suspending at or fixing onto a wall to give an excellent antitheft protection effect.