Abstract: Traditional price indexes, such as CPI and central CPI etc, essentially calculated using the formulae of chain base index based on Laspeyres, therefore, none of these indexes is much valid for measuring the movement in the general price level. This paper proved in theory that there are three factors caused lack validity:the ratio of percentage of new-product/percentage of obsolescent-product, the ratio of price of substitutes/the price of the product, and the change of consume construct explained by the Engle-Low.