Understory vegetation plays crucial roles on nutrient turnover and cycling in plantation soil by regulating soil microbial biomass and extracellular enzyme activity. Under normal condition, the understory vegetation generally contains diverse plant species in plantations. Whether increasing the species abundance of understory vegetation can change the effect of understory vegetation is still inconclusive. In this study, a diversity gradient of understory vegetation species was setup to study its effects on soil microbial biomass carbon (MBC) and nitrogen (MBN), soil microbial communities' metabolic profiles, and enzyme activities that related to soil C and N turnover in a popular plantation. Three understory treatments were designed including the understory vegetation removal, the retention of single main understory vegetation species, and the retention of natural diverse understory vegetation. The soils were sampled in June, August and October after one year treatment for the analysis of corresponding soil microbial properties. The results showed that the understory vegetation treatments had strong effects on 0-5 cm soil layer in August. Compared with understory vegetation removal, the retention of single main understory species resulted in significant increases in soil MBC and MBN contents and the activities of soil β-glucosidase, polyphenol oxidase and arylamidase by 27.91%, 54.48%, 14.74%, 32.53%, and 6.20%, respectively, in the 0-5 cm soil layer in August; moreover, the retention of natural understory vegetation showed further increases by 4.88%, 14.93%, 9.22%, 13.63%, and 12.86%, respectively, when compared with the retention of single main understory species. Understory vegetation treatments also changed soil microbial communities' metabolic profiles, soil microbial Shannon index of the 0-5 cm soil layer in August increased significantly with the increase in understory vegetation species. The main carbon sources utilized by soil microorganism with understory removal treatment were a few kinds of carbohydrates, amino acids and esters. By contrast, soil microorganisms under the retention of single main understory species treatment improved the utilization capacity of the above-mentioned carbon sources, and expanded the available carbon sources to some organic acids. At the same time, the retention of natural diverse understory vegetation enabled soil microorganisms to effectively utilize all of the 31 carbon source types. Therefore, the retention of understory vegetation, especially with high diversity, was beneficial to increase soil microbial biomass, improve soil microbial metabolism and activities, and to a certain extent accelerate the decomposition of soil organic matter and nutrient cycling.