Abstract: In this paper, the principle of design and constitution of the installation for measuring photosynthetic rate of plant population, and methods of measurement and calculation were introduced. This installation with semi-closed measurement system and CO₂ infrared gas analyser, can measure and control change of CO₂ concentration continuously in an assimilation chamber. Its measurement range is: 0～1000 ppm; resolution; 2 ppm. Good results by using this installation to measure photosynthetic rates of wheat, corn, soybean and Chinese cabbage under different concen-trition of CO₂ had been achieved.