Tree-ring density (i.e., wood density) is one of the most important proxies for climate reconstruction and environment change study. Generally, seven parameters of tree-ring data (i.e., earlywood and latewood widths, mean earlywood and latewood densities, maximum and minimum densities, and total tree-ring widths) can be obtained by tree-ring density measurement. Tree-ring density, especially maximum latewood density (MXD) has been widely used on summer temperature reconstruction and volcanic eruption study in the North Hemisphere. Climate warming in the 20~(th) century and the anomalous temperature increase in the recent period have been suggested by many temperature reconstructions based on tree-ring density data. Over the past decade,tree-ring density research experienced a fast development in China and great advances have been achieved. Some studies indicated that tree-ring density, including mean earlywood density, minimum earlywood density and MXD in several tree species in the Northwestern China were correlated with precipitation in spring or annual precipitation. But, MXD in the Northeastern China and the Tibetan Plateau significantly correlated to summer temperature variability. Thus, precipitation reconstructions based on tree-ring density data have been performed in the Northwestern China and summer temperature variability has been reconstructed both in the Tibetan Plateau and the Northeastern China. The variance explained by the model in the instrumental period is relatively higher in the Tibetan Plateau than in Northwestern and Northeastern China. Additionally,tree-ring density data also were casually used in the other areas. For example, glacier mass balance reconstruction, air pollution and so on. In this paper, the methods used for measurement of tree-ring density (e. g., X-ray measurement, high-frequency densitometry, blue reflectance and so on) are summarized,and the important results obtained based on tree-ring density in the North Hemisphere and China are reviewed. After a statement of the critical and controversial questions for tree-ring density research,the potential importance and availability of tree-ring density data are clarified, providing a reference for the tree-ring density researches in the future.