Abstract: Green manure planting is an ideal technique for weed control. A field experiment was conducted to study the effects of different planting densities of the green manure Macrotyloma uniflorum on weed communities and coffee growth in young coffee plantations. Five months after planting green manure, the effects of five planting densities of M. uniflorum S0: 0 kg·(667 m²)^-1; S1: 0.5 kg·(667 m²)^-1; S2: 1.0 kg·(667 m²)^-1; S3: 1.5 kg·(667 m²)^-1; S4: 2.0 kg·(667 m²)^-1 on weed communities and coffee growth were investigated. The results show that with the increase in M. uniflorum planting density, the importance value of the dominant weed Cyperus rotundus significantly decreased, and the height, density, coverage, and biomass of weed community and the dominant weed C. rotundus were all lower than those of S0. As the planting density of M. uniflorum increased, the Margalef richness index and Shannon-Wiener diversity index of the weed community first decreased and then increased, the Simpson dominance index gradually decreased, and the Pielou evenness index gradually increased. Under the M. uniflorum planting treatment, the response ratios of height, density, coverage and biomass of weed community were significantly lower than those of S0. M. uniflorum planting significantly reduced the response ratio of the Simpson dominance index and significantly increased the response ratio of Pierou evenness index. Among the functional groups, the importance value of Poaceae was significantly higher in S2 and S3 than in S0, while the importance values of other functional groups showed no significant differences. There was no significant correlation between the importance value of weed functional groups and community diversity indices. After planting M. uniflorum, the plant height, crown width, ground diameter, leaf length, leaf width, number of primary branches, length of the longest primary branch, number of internodes on the longest primary branch, leaf thickness of coffee were all higher than those of S0, with leaf width reaching a significant level. Overall, M. uniflorum planting promoted coffee plant growth, had a significant control effect on weed communities and the dominant weed C. rotundus, and significantly reduced the dominance of weed communities without affecting the richness and diversity of weed communities in the short term. It is recommended to use a planting density of 1.0 kg·(667 m²)^-1 for M. uniflorum in young coffee plantations to achieve higher economic and ecological benefits.