The inhibitions of scenedesmus obliquus, chlorella pyrenoidosa and chlorella vulgaris stressed by the same concentration of Hg2+, Cd2+, Cu2+, Zn2+ are investigated with pulse amplitute modulation (PAM) technology.The chlorophyll-a fluorescence parameters of PSⅡ system are measured and the inhibition rates and response times of three kinds of algals offer reference for the sensitive microalgal species. From the results, chlorella pyrenoidosa is determines as a sensitive microalgal species to Hg2+, Cd2+, Cu2+, Zn2+. The sensitivity of three microalgals can be expressed as chlorella pyrenoidosa>scenedesmus obliquus>chlorella vulgaris.The toxicity results of four kinds of heavy metal ions can be expressed as Hg2+>Cu2+>Cd2+>Zn2+. Compared to algal growth inhibition experiment, this new method for a sensitive microalgal specie selection is simple and rapid.