Objectives:As a indicator of the bioassay technology with ToxY-PAM,the states of growth and biological activity of phaeodactylum tricornutum were key factors for the process of biological detection.To obtain the optimization of culture conditions for phaeodactylum tricornutum,the influence of light intensity,temperature,the concentration of nitrogen and phosphor in culture medium were studied.Methods:The number of cells was counted with hemacytometer and the optimal conditions of culture selecled by comparing relative growth rates under different conditions of culture.Results:The optimal conditions of culture of phaeodactylum tricornutum were: 2000Lux in light intensity,20℃ in temperature,12 mg/L nitrogen and 1.3 mg/L phosphor in culture medium.Conclusion:Under above optimal conditions,phaeodactylum tricornutum grew fast and had a better biological activity.Inhibition of photosynthesis of phaeodactylum tricornutum produced by toxicants in water could be shown exactly.Phaeodactylum tricornutum in these conditions could be used as the indicator of the bioassay technology with ToxY-PAM.