Abstract: Photosynthetic rate of algae, expressed as oxygen evolution per unit chlorophyll a and one hour, was used as an index of chemical compounds toxicity.It is obvious that the toxicity can be measured exactly, briefly and quickly; and can be carried out anytime to detect the process of photosynthesis.In comparison with the routine acute toxicity test of algae, the temporal scale of whole test range is reduced from 96hr to 2 hr.And the sensitivity was promoted about one time.The 50% of Effective Concentration (EC50) of Cu2+ on algal photosynthesis is 0.105 mgl-1, only about one half of that in acute test of Cu2+ on algal toxicity .