A lamp driving device includes a DC power supply, a square wave switch, a square wave controller, a plurality of lamps, a starting transformer, and a common transformer; a plurality of starting transformers and a plurality of common transformers are disposed at both sides of the plurality of lamps respectively, the plurality of starting transformers and the plurality of common transformers can have their primary sides or secondary sides cascaded and connected to the square wave switch; the present invention can effectively control the output power to the lamps, reduce the size and the temperature of the device, and improve the overall efficiency by connecting the primary sides or secondary sides in series.