Under-reporting rate of China’s 2000 population census stood at 1.81%,indicating deterioration of demographic data quality in China.On the one hand,this rate is an objective representation of current data quality;on the other hand,China’s population data quality is good as compared to other countries.Causes for under-reporting are many-sided,including design of census plan,people’s cooperation,funds input,administrative examination etc.As for the coming 6th national population census,we need to seriously analyze the causes of under-reporting,so that we can take appropriate measures to improve census data quality.