Population density is the number of people living in a unit of land, and it is an indicator of the density of population around the world. However, population density only reflects the general quantitative relationship between population and land area, and cannot reflect the composition of the population on a unit of land. It reflects the relationship between the population and the land area that actually feeds him. In order to make up for these shortcomings, some new complementary density indicators have appeared, such as physiological density, agricultural density, etc. , and the population density calculated by area is called the mathematical density of the population. The text is intended to explain:. ['Reasons for the evolution of the meaning of population density.', 'The generation process of complementary density indicators.', 'Specific manifestations of population density.', 'Relationship between land area and population composition.']