Abstract: Photosynthetic characteristics of sunflower and its responses to different ecological factors were studied.The results indicate that the light saturation point for the plant is 1 700 μE·m^-2·s^-1;the light compensation point ranges from 95 μE·m^-2·s^-1 to 120 μE·m^-2·s^-1;the photosynthesis rate increases with the light intensity within the extent of the light saturation but decreases with increase in the relative humidity.Temperature is the main factor limiting light utilization and the most suited temperature for growth is 23～33 ℃;and it is accumulation of the photosynthetic organic matter,soluble sugar,that causes "midday dormancy",rather than stomatal conductance.