We investigated the understory species diversity, soil physicochemical properties, and their relationships with a Cupressus funebris plantation using typical plot methods under five different stand densities (1,100, 950, 800, 650, 500 tree/hm²) in the Yunding Mountain, Chengdu, to find a density that could maintain a stable community structure. The results identified a total of 168 vascular plant species belonging to 75 families and 140 genera. With a decrease of stand density, the shrub layer diversity indexes initially increased and then decreased. The highest species diversity appeared at a density of 650 tree/hm². The herb layer diversity indexes showed a double-peak distribution:initially increased, then decreased, and increased again, then finally decreased. The turning point appeared at densities of both 950 and 650 tree/hm². Meanwhile, we found that the sunlight played an important role affecting the species richness index variation. Our results also suggested that different stand densities had significant effects on the contents of total nitrogen, total potassium, available potassium, and organic matter in the soil. The responses of organic matter, total nitrogen, total potassium, and available potassium were similar to the shrub layer diversity index variation. Moreover, there was a significant correlation among potassium, nitrogen, organic matter, and understory species diversity. The results indicated that a stand density of 650 tree/hm² would optimally enhance the understory species diversity and soil fertility.