Abstract: In order to assess the bioenvironmental effect of a mixed herbicide which composed by Fenoxaprop-ethyl and Cyhalofop-butyl correctly, the single toxic and joint toxic effects of them on Lemma Paucicostata were studied. The results showed that the IC50 of Fenoxaprop-ethyl and Cyhalofop-butyl to Lemma Paucicostata for 96 h were 3.33 and 5.04 mg/L respectively, the IC50 of joint toxicity of them for 96 h to Lemma Paucicostata was 3.14 mg/L. The additive index (AI) was 1.566, the combined toxicities were the synergism action. The content of chlorophyll a and chlorophyll b of Lemma Paucicostata both affected by the single and joint toxicity of Fenoxaprop-ethyl and Cyhalofop-butyl with the concentration increased.