The toxic tests of surfactant LAS(Linear Alkylbenzene Sulfonate) was carried out at three biological organization levels species populatiocommunity Results showed that there were no specific differences in the acute toxicity of LAS among five protozoan species. Paramecium caudatum, Tetrahymena americanis, T. pigmentosa, T.borealis, 3IV WWO.T.borealis 3II VM665, and T.tropicalis DIV TC89 (P > 0.05). 12h-LC50 was between 7.0 and 10.47 p pm. At the population leve of Tetrahymena americanis, the 12h-LC50 was 17.21 p pm; the effective concentrations of LAS. based on the respiration of Tetrahymena americanis, were 12.84 p pm and 9.63 p pm (P < 0.05). as determine by using the Gilson Differential Respirometer and the polyrographic Respirometer respectively. From low (3p pm) to high (40p pm) concentrations, the toxic effects of LAS on the structure and function of microbial community increased. The structural parameters included the number of protozoan species and their composition. The functional parameters included the protozoan colonization (Seq. G. T90%) and the ratio of photosynthesis to respiration. There was a strong negative correlation between Seq and natural logarithm of LAS concentration with the regression formula of Seq = 39.7281-5.6722 lnC, P < 0.05. The effective concentrations (EC) were calculated by the formula, thus the EC_(50), EC_(20), EC_5 were 4.684, 0.51, 0.1681 p pm respectively. Based on LC50 in species and population levels, MATC were 0.71 and 1.721 p pm when AF was 0.01. Compared the MATC and EC_(20), EC_5, it showed that the results from the microcosms tests have more scientific and environmental realism. Thus, to the Donghu Lake. the MATC must be adjusted below 0.5p pm. below 0.17p pm is even better.