Abstract: The density of the sand is important factors for their engineering properties, the evaluation methods of sand density is more, but the results may have differences, usually means, this is testing error that is caused by different testing method. In fact, the causes of the sand formation and buried conditions, groundwater is also the important factors. The distribution of effective stress in the soil will be changed by the pressure of the confined water, and the anomaly of sand density may he formed. Project should focus on the analysis of the formation reasons for sand density, according to theoretical knowledge, to identify applicable conditions of various evaluation methods and their advantages and shortcoming.