Abstract: with the acidification and increased pH caused by the outbreak of red tides in coastal water,the effects of pH on the marine algae physiological and ecological impact have caused human's concern gradually.In this experiment,we use Karenia mikimotoi as research object to study the effects on its physiological state in different pH.The results showed that pH significantly affect the growth,nutrient absorption,chlorophyll concentration,photosynthetic rate and affinity for inorganic carbon of the alga K.mikimotoi.When pH = 6.2,the cell growth was inhibited.Growth rate,nutrient absorption,maximum photosynthetic rate and affinity for inorganic carbon are the lowest; When pH = 8.2,the growth of cells,absorption of nutrients and highest maximum photosynthetic rate are the fastest; pH = 7.2 and 9.2,the algal cells can still maintain higher growth and higher affinity for inorganic carbon is larger than those in pH = 8.2.In short,high or low pH stress can cause adverse changes in the physiological state of the alga K.mikimotoi,especially when the pH is too low.