The invention provides a conversion method of three-color data to four-color data, including steps: A) converting input RGB data to intermediate RGBW data according to first, second and third predetermined saturation parameters; B) obtaining first, second and third saturation adjust parameters according to the intermediate RGBW data and standard RGBW data; C) using the first, second and third saturation adjust parameters to respectively adjust the first, second and third predetermined saturation parameters; D) using the first, second and third predetermined saturation parameters after being adjusted to convert the input RGB data to output RGBW data. The invention further includes a conversion system of three-color data to four-color data.