Abstract: Through the experiment of water train in laboratory ,the effect of Cu,Cd,As and their compound pollution on the chlorophyll and MDA（Methane Dicarboxylic Aldehyde） concentration in the duckweed were studied. The results show that at the same concentration level,the single-toxicity order of Cu, Cd,As to the chlorophyll of duckweed is Cd＞Cu＞As,Cd≈As＞Cu is the MDAs single-toxicity order.Compared with the single pollution,the eco-toxicity of compo und pollution of As+Cd and Cu+Cd becomes stronger, which presents a synergistic effect, as far as the lethal concentration to the cell of duckweeds lamina is concerned;As+Cu and As+Cu+Cd is inclined to an antagonistic effect of which the eco-toxicity of compound pollution becomes weaker. The eco-toxicity of duckwee ds MDA is relatively complex.Besides As+Cd, the other compound pollution has an effective over-penetration on the cell membrane of duckweed.