Cell density, morphology and characteristic change of Euglena sanguinea and its relationship with environmental factors were investigated during algal bloom and decay process in Beijing Cuihu Lake National Wetland Park. The phytoplankton and water samples were collected from August to November, 2016. The results showed that the density of Euglena sanguinea increased steadily from late August to mid-September, and reached the peak point on September 20(5.13×10⁶ cells/L). Then it decreased significantly from late September to the end of November. During the decay process of Euglena sanguinea, the cell size became smaller, and the cellular morphology transformed from spindly to round. The cell color changed from dark to light. Meanwhile the speed of its movement gradually slowed down. Eventually the cell formed into hypopus with the spores in it covered by a transparent gum. The results indicated that the decline of Euglena sanguinea density was significantly depended upon water temperature and pH. The threshold ranges of optimum temperature was 23~27℃, that of pH was 7.3~8.0. The stratification phenomenon of algae density was mainly affected by light intensity. The density of algae was relatively low in the weak light region. The decay process of Euglena sanguinea was positively correlated with ammonia concentration, and it could accelerat decay speed at low concentration. Furthermore, the decay process of Euglena sanguine was negatively correlated with total nitrogen concentration. It significantly increased during the process of decline. It was less affected by total phosphorus concentration. There was no significant correlation between the decay process and dissolved oxygen.