Abstract: Constant luminance principle is the keystone of the color television system. It not only decides that television image signal designs use the combined framework of luminance signal Y and chroma signal C, but also decides that the television design techniques use circuit setting of luminance channel and chroma channel.As the technology of television develops, the research on the invalidation of Constant luminance principle seems more attracting. As early as the mid twentieth century, it is found that the nonlinear of kinescope would invalidate the Constant luminance principle. The research of this paper also points out that when transmitting the chromatic images, the mistaken of G-Y demodulator would also cause invalidation of Constant luminance principle.