The present invention discloses a signal conversion device, a signal conversion method and a display device. The signal conversion device includes a gamma conversion unit, a brightness detection unit and a brightness processing unit, wherein the gamma conversion unit is used for performing a gamma conversion process on RGB input signals and generating RGB brightness input values; the brightness detection unit is used for generating a W brightness input value based on RGB proportional coefficients and the RGB brightness input values; and the brightness processing unit is used for generating RGBW output signals based on the RGB proportional coefficients, the RGB brightness input values and the W brightness input value. With the present invention, the brightness of a displayed image can be increased without increasing the power consumption, so that the contrast of the displayed image is increased, and the display quality of the image is also improved.