Abstract: A backlight device includes a number of light emitting strings, a number of switch units, a first control unit and a second control unit. Each of the light emitting strings includes a number of light emitting units connected in series. The light emitting strings are configured to be driven by electric power to emit light. One of the switch units serves as a reference switch unit. The first control unit is configured to adjust a voltage level of the control terminal of the reference switch unit according to a voltage level of the reference node. The second control unit is configured to instruct the first control unit to set the voltage levels of the control terminal of each of the switch units and the electric power according to pre-calibration data corresponding to the illumination type of the backlight device.