An RGB to RGBW brightness compensation method and device is disclosed. The method includes: retrieving multiple gray scale values of RGB color RGB(x, y, z), wherein x, y, z are gray scale levels of red, green and blue sub-pixels of an RGB model, 0≤x, y, z≤1; multiplying the gray scale values of RGB color RGB (x, y, z) by N to obtain an enhanced gray scale values of RGB color RGB (Nx, Ny, Nz), wherein N is 4/3 and a minimum value selected from x, y, z is taken to represent a first conversion value W1; and obtaining multiple gray scale values of RGBW color RGBW (Nx−W1, Ny−W1, Nz−W1, W1) according to the enhanced gray scale values of RGB color RGB (Nx, Ny, Nz) and the first conversion value W1. Accordingly, a brightness of the RGBW model of a liquid crystal panel is compensated.