An illuminating system includes a universal lamp holder. The universal lamp holder includes a switch unit, a starter, a ballast, a first socket and a second socket. The ballast is connected to the voltage-input terminal and the first socket. The starter is connected to the first socket and the switch unit. The second socket is connected to the switch unit and the voltage-output terminal, and the switch unit is configured for selectively connecting the second socket to the voltage-input terminal or connecting the second socket to the starter. When the second socket is switched to connect to the voltage-input terminal, the illuminating system is switched to an LED lamp mode, and when the second socket is switched to be connected to the starter, the illuminating system is switched to a fluorescent lamp mode.