Rainfall is the main source of soil moisture in the Loess Plateau, China. In order to investigate the impact of rainfall reduction on soil nutrients and soil microbial communities of a Robinia pseudoacacia plantation in the sub-humid area of Loess Plateau, a rainfall reduction experiment was conducted. Rainfall exclusion was conducted by installing translucent panels to divert part of the throughfall outside the treatment plot. Rainfall partitioning investigation and the panel area for throughfall exclusion estimated that the treatment plot received about 47% reduction of the total rainfall input. After 4 years of treatment, soil nutrient content, soil microbial community structure and diversity were measured in the sample plots with and without rainfall reduction. The main results are as follows:(1) The contents of soil organic carbon, total nitrogen, and total phosphorus in the treated plot were lower than those in the control plot, and the difference in the total nitrogen content reached a significant level (P<0.05). (2) There was no significant difference in soil microbial diversity between growing and non-growing seasons in the control plot. The Simpson index of the treated plot during non-growing season was significantly lower than that in growing season and that of the control plot in non-growing season (P<0.05). Rainfall reduction had a significant effect on the community structure of bacteria and fungi (P<0.05). The rainfall reduction treatment significantly decreased the relative abundance of Mcidobacteria, but increased the relative abundance of Actinobacteria, Chloroplexi, Ascomycota, and Mortierellomycota (P<0.05). (3) Both soil organic carbon and total nitrogen contents were significantly correlated with the relative abundance of some bacteria, and soil total nitrogen content was significantly positively correlated with the relative abundance of Mortierellomycota fungi (P<0.05). In brief, the 4-year treatment of rainfall reduction resulted in a weak impact on soil nutrient content, but had significant effects on bacterial diversity, and the structure of bacterial and fungal community. There was also a correlation between soil nutrient content and the relative abundance of soil microorganisms. The results provided theoretical reference for further study on the impact of potentially continuous rainfall reduction on forest ecosystems in the sub-humid area of Loess Plateau, China.