We used a typical sampling method to study the impact of five types of stand densities (A: 1000 trees/hm²; B: 1125 trees/hm²; C: 1250 trees/hm²; D: 1375 trees/hm²; E: 1500 trees/hm²) on the species diversity and biomass of understory plants and the correlation between the species diversity and biomass of the understory plants in a Pinus massoniana plantation at Wenfeng Mountain, Xinjin County, Sichuan Province. The results were as follows: (1) A total of 124 plants, belonging to 74 families and 115 genera, were investigated in three layers. The shrub layer had fewer species than the herb layer. (2) The dominant species in both shrub and herb layers under different stand densities were nearly consistent. (3) The species richness index (D), Shannon-Wiener index (H) of the shrub layer, and four diversity indexes of the herb layer showed maximum and minimum values in densities B and E, respectively. The D value of the shrub layer initially increased and then decreased with an increase in density, but the changes in the other three indexes were not obvious. The species evenness index, J_sw, of the shrub and herb layers showed few differences among the various densities. (4) The shrub layer had more biomass than the herb layer. In general, the aboveground biomass of the shrub and herb layers was higher than the underground biomass, and they both were maximum at density B. The underground biomass of the shrub layer remained stable, whereas that of the herb layer changed greatly. (5) In addition to the negative correlation between the J_sw value and biomass of the shrub layer, all other indicators showed positive correlations. Each stand density had different effects on the understory plant diversity and biomass of the P. massoniana plantation. After comprehensive analyses, stand density of 1125 trees/hm² was found to be more conducive to sustainable and healthy development of the P. massoniana plantation.