Species diversity: other measures of species diversity include the rarity of populations and the number of evolutionary rare features they possess. The abundance of species is inversely proportional to latitude. The farther away from the equator, the rarer the species is, even if the reduction of surface area in high latitudes is taken into account. There is also a relationship with altitude, such as the vertical distribution of species. Genetic diversity is the basis of species diversity. Species diversity is the center of biodiversity and the most important structural and functional unit of biodiversity. It refers to the richness of animals, plants, microorganisms and other biological species on the earth. Species diversity includes two aspects: on the one hand, the richness of species within a given region, which can be called regional diversity of species, and, on the other hand, the degree of uniformity of species distribution in ecology, It can be called ecological diversity or community diversity. Species diversity is an objective index to measure the abundance of biological resources in certain areas. It is measured by the number and distribution of genetic diversity of species in a certain spatial range at multiple levels. In general, the larger a species's population, the greater its genetic diversity. However, the population increase of some species may lead to the decrease of other species, which leads to the decrease of species diversity in a certain region. Three typical forest vegetation types of Mulan Forest Bureau in Weichang County were studied. The species number and diversity were compared from the Arbor layer, Irrigation layer and herbaceous layer. The results showed that there were 56 species in mixed forest of Larix gmelini, 53 species in birch forest and 44 species in mixed forest of Pinus tabulaeformis and Quercus mongolica. The species diversity of Arbor layer was not obvious. The biodiversity index of shrub layer was generally mixed forest of Pinus tabulaeformis and Quercus mongolica > mixed forest of Populus davidiana and birch > mixed forest of Larix gmelinii and birch. In addition to the Simpson index, the herbaceous layer biodiversity index was the same as that of Pinus tabulaeformis and Quercus mongolica mixed forest > poplar birch mixed forest > larch birch mixed forest.