Mountainous areas in China account for two-thirds of the total land area. Due to rapid urbanization, rural population emigration in China's mountainous areas is very significant. This raises the question to which degree such population emigration influences the vegetation greenness in these areas. In this study, 9,753 sample areas (each sample measured about 64 square kilometers) were randomly selected, and the influences of population emigration (population pressure change) on vegetation greenness during 2000–2010 were quantitatively expressed by the multivariate linear regression (MLR) model, using census data under the condition of controlling the natural elements such as climatic and landform factors. The results indicate that the vegetation index in the past 10 years has presented an increasing overall trend, albeit with local decrease in some regions. The combined area of the regions with improved vegetation accounted for 81.7% of the total mountainous areas in China. From 2000 to 2010, the rural population significantly decreased, with most significant decreases in the northern and central areas (17.2% and 16.8%, respectively). In China's mountainous areas and in most of the subregions, population emigration has significant impacts on vegetation change. In different subregions, population decrease differently influenced vegetation greenness, and the marginal effect of population decrease on vegetation change presented obvious differences from north to south. In the southwest, on the premise of controlling other factors, a population decrease by one unit could increase the slope of vegetation change by 16.4%; in contrast, in the southeastern, northern, northeastern, and central area, the proportion was about 15.5%, 10.6%, 9.7%, and 7.5%, respectively, for improving the trend of NDVI variation. With the rapid advance of industrialization and urbanization, massive rural residents migrated into urban areas, and rural population decreased by about 17% at the national level. The existing literature could not identify the types, intensity, and contribution ratios of human activities that influence vegetation change and do not exclude the influences of natural factors, such as temperature and precipitation. This study focused on changes in population pressure in mountain areas and used the multiple regression model to quantitatively assess the effects of human activities on the vegetation greenness, based on the spatial rasterization for all explanatory variables. We found out that population decrease influenced vegetation greenness significantly in China; The marginal effect of population decrease to vegetation change presented obvious difference: In the southwestern, a population decrease by 1 unit could increase by 16.4% of the slope of vegetation change; in the central area, the proportion was only 7.5%.