A conversion system for converting RGB data to RGBW data includes a four-color determination unit configured to determine outputted RGBW values according to inputted RGB values and a color saturation value calculated from the inputted RGB values; and an output unit configured to output the outputted RGBW values. A conversion method for converting RGB data to RGBW data is also disclosed. The conversion system and method for converting RGB data to RGBW data of the present invention can appropriately decrease the outputted W value and appropriately increase the outputted RGB values at the same time. Comparing to the conversion system of the display device of the conventional technology based on four-color subpixels, the brightness of the display device of the present invention can be maintained in the same degree, and increase the color saturation of the display device of the present invention.