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 .