A method and device for image conversion from RGB signals into RGBW signals, after converting the RGB luminance input values into RGBW luminance output values, determine color-cast-removed RGBW luminance output values respectively, according to a position relationship between RGBW color coordinate values to which the RGBW luminance output values correspond respectively and a predetermined actual color coordinate value of a color having monochromatic color cast among RGBW in a chromaticity diagram, and thereafter convert the color-cast-removed RGBW luminance output values into corresponding RGBW output signals respectively and output the same. By means of the above method provided in the present disclosure, color having monochromatic shifting among RGBW can be compensated back to the expected RGBW color coordinates and luminance values, so the problems of color gamut deviation and color distortion caused by RGBW monochromatic color cast are eliminated, color gamut of the displayed image is more accurate.