This paper studied the biomass, species composition, and diversity index of artificial grassland plant communities with different establishment duration in the source region of the Three Rivers, and examined the relationships of soil physical and chemical properties with the changes of the plant community biomass and species diversity. The results showed that the species composition, functional group composition, and quantitative characters of the plant communities varied greatly. Soil moisture content increased with increasing species diversity, while soil bulk density was in adverse. Soil microbial biomass carbon significantly positively correlated with soil moisture and organic matter contents, but negatively correlated with soil bulk density. Soil organic carbon content had a "V" type change, which was consistent with the change pattern of soil moisture content, and decreased with increasing soil bulk density. Plant community biomass had significant positive correlations with the contents of soil nutrients and moisture, and the increase of the above- and below-ground biomass of plant communities promoted the increase of soil nutrient contents.