Abstract: This study investigated the effect of light intensity, pH and temperature on the photosynthesis of Potamogeton crispus. At a given temperature, net oxygen production of P. crispus was linear related to light intensity. Light compensation point increased with increased temperature. In the natural environment of P. crispus, below 30℃ an increase in temperature enhanced photosynthesis of P. crispus. At high pH (>10), shortage in carbon source had great influence on the photosynthesis of P. crispus. Combinations of high pH and strong light intensity seriously affected photosynthesis. High temperature and deficiency in nitrogen and phosphorus are not major cause for the mortality of P. crispus. The combination of infavorable light conditions (inhibition by strong light at surface and light deficiency at deeper depths) and shortage of inorganic carbon source at high pH may have caused summer mortality of this species.