Understanding the main factors that affect the productivity of Eucalyptus plantations is important for promoting productivity. To this end, we collected density, age, altitude, mean annual temperature mean annual precipitation, and tree biomass data from Eucalyptus plantations in the three Eucalyptus plantation areas in China, which are Guangdong Province, Guangxi Province, and Yunnan Province. Principal component analysis and mixed-effects modeling were then used to explore the influences of various factors on the productivity of the Eucalyptus plantations. Mean annual temperature, mean annual precipitation, age, altitude and density were the main factors that affected the biomass of Eucalyptus plantations in the three provinces which mean annual precipitation (P < 0.01), forest age, and density (P < 0.05) had significant effects, due to the three provinces were located in a subtropical region. The temperature of all three areas is suitable for Eucalyptus growth. However, the areas are also subject to distinct dry and wet seasons, which means that the distribution of precipitation is uneven, both spatially and temporally, which subsequently causes significant differences in the Eucalyptus biomass of the three provinces. Therefore, mean annual precipitation is a key factor in Eucalyptus biomass in China.