A field experiment was carried out to study the damage of Eupatorium adenophorum Spreng to garlic yield and its economic thresholds. The optimum regression model for the garlic yield loss and the density of Eupatorium adenophorum Spreng weed was established and the competitive mechanisms of nutrients and water to affect effecting garlic yield were discussed. The results showed that the garlic yield decreased significantly with the increasing density of weeds. The hyperbola model(1/y=-0.018 286+0.435 75/x，F=261.79**) could well fit the relation between the garlic yield loss(y) and the weed density(x). The economic infestation levels for the Eupatorium adenophorum Spreng by manual weeding, 10% Glyphosate and 24% Picloram were 2.630%, 0.277% and 0.369% respectively, and the economic threshold were about 1.09, 0.12 and 0.16 plant per m2 respectively.