Abstract: To understand the hazards of triazole fungicides on aquatic ecosystems, the author investigated the growth inhibition of 17 kinds of triazole fungicides on Chlorella phrenoidosa according to the OECD 201 method. The results showed that: the toxicity of different fungicides was quite different. Flusilazole and myclobutanil were very toxic compounds with the highest toxicity to Chlorella phrenoidosa. Flutriafol, cyproconazole, triadimenol, tricyclazole, poclobutrazol, uniconazole and epoxiconazole were harmful compounds with the lower toxicity. Other triazole fungicides were toxic compounds. The results of this study provided the scientific basis for security applications and risk assessment of triazole fungicides.