Disclosed is a primary color conversion method, which may expand primary color signals that are applicable to a display device. The primary color conversion method includes: acquiring color coordinates and a brightness value of a first color in a first color gamut having M primary colors according to gray-scale values of the M primary colors corresponding to the first color; and mapping the color coordinates and the luminance value of the first color in the first color gamut to color coordinates and a brightness value of a second color corresponding to the first color in a second color gamut having N primary colors. 3≤M, 3≤N, M is different from N, and M and N are positive integers.