Abstract: The aim of this study was to determine the effect of common leaf spot on the photosynthetic performance and forage quality of alfalfa, and to evaluate the importance of this disease. Five indices of photosynthetic performance were measured in diseased and healthy leaves using an LI-6400 photosynthetic analyzer, and the concentrations of 10 nutrients were determined in diseased and healthy leaves and stems. The results showed that the ash, acid detergent fiber (ADF), neutral detergent fiber (NDF), calcium (Ca), tannin, and total phenols content in diseased leaves was 19.94%, 71.21%, 39.68%, 136.42%, 21.93% and 50.55% higher, respectively, than their corresponding contents in healthy leaves; the correlation coefficient for each of these indexes with disease severity was 0.95, 0.95, 0.91, 0.85, 0.89, and 0.85, respectively. Crude protein (CP) and potassium (K) content was decreased by 15.67% and 29.91%, respectively, in diseased leaves, and their respective correlation coefficients with disease severity were -0.99 and -0.86. Crude fat (EE) and phosphorus (P) contents were decreased in diseased leaves, but they were not significantly correlated with disease severity. Correlations between nutrient indexes and disease severity were not determined for stem material, but the ash and NDF contents were lower in diseased stems than in healthy stems, while other nutrients showed the same trends as observed in diseased leaves. Compared with healthy leaves, diseased leaves showed a higher intercellular CO₂ concentration (C_i), but this was not correlated with disease severity. The net photosynthetic rate (P_n) and transpiration rate (E) of diseased leaves decreased by 93.27% and 68.34%, and their correlation coefficients with disease severity were -0.82 and -0.86, respectively. Stomatal conductance (G_s) and water use efficiency (WUE) tended to be lower in diseased leaves than in healthy ones, but they were not significantly correlated with disease severity. There was no significant correlation between leaf P content and photosynthetic parameters; however, there were extremely significant or significant correlations between other quality indices and P_n and E. In a principal components analysis of leaf quality and photosynthetic performance data, principal component 1 accounted for 80.94% of data variance and showed positive coefficients for ash, ADF, NDF, Ca, tannins, total phenols, and C_i, and negative coefficients for CP, EE, K, P_n, E, G_s, and WUE. In conclusion, common leaf spot negatively affects the quality and photosynthesis of alfalfa, and also affects the interaction between photosynthetic performance and forage quality.