Plant interactions are one of the fundamental processes shaping the structure and function of plant communities and help create species diversity. Species diversity affects the current functioning of ecosystems and their resistance and resilience to future climate change. In harsh environments such as drylands, positive plant-plant interactions are important in promoting species diversity. Juniperus sabina is an evergreen shrub that is native to the semiarid areas of northern China. Because J. sabina can modify some harsh environmental conditions in its role as a nurse plant, it is expected to facilitate species diversity, although it may exhibit allelopathic inhibition. Previous research has only examined effects of J. sabina coverage on alpha-diversity in a single-year, and its effects on the beta-diversity of the plant community structure in the local ecosystem are still unclear. We compared environmental conditions and plant species composition inside and outside of 11 J. sabina patches to evaluate the effects of its coverage on the species diversity of the understory community structure through modifying microhabitat conditions. Water and nutrient conditions were higher inside the patches, whereas light conditions were higher outside. More perennial herbs and C3 plants were found inside and more annual herbs and C4 plants were found outside. There were different trends in alpha-diversity each year, while beta-diversity was consistently greater inside the patches. This research suggests that the coverage of J. sabina can drive different community structures by providing heterogeneous environmental conditions, and would increase plant species diversity in the local ecosystem.