The change of relative difference and absolute difference of per capita income is a relative and different reciprocal U type. The condition of kink-appearing is that the increase of per capita income in undeveloped area must surpass that of developed area. Since the carrying out western development, the two differences of per capita income tend to increase, which reflects the difficulty and durability of western development on the one hand and challenges the realization of expected objective of western development. Thus, the implementation of western development must be greatly consolidated.