This study conducts projection to the size and age structure of 2010 population census and obtains simulative demographic indices since 1990.Comparing the simulated with statistics from other sources,it turns out that population estimation and projection in past 20 years shared a common defect of which birth number,population growth and fertility level were badly over-estimated.As a result,the population ageing has been significantly underrated.Some adjusted indices produced even larger bias than the original ones,and this mislead to a wrong direction.Such phenomena reflect strong biased orientation in demographic study for the past 20 years.