Abstract: The photosynthetic growth model for plantlets in vitro is the basis for its micro-environment control, the measuring system for photosynthesis rate of plantlets in vitro is the vital equipment for developing the growth model. There are some existing measuring systems for photosynthesis rate of plants, such as Li-6400, but they cannot be used for plantlets in vitro. In this paper, based on the analyses of the existing equipment and the consideration of the feasibility in type choice in the large-scale facilities for plant tissue culture, a half-opened measuring system for photosynthesis was designed and developed. It can work in real time automatically and reliably and continuously, and has no disturbance to the micro-environment. The data from the system can reveal the plantlets growth in truth, and so it can be used directly in micro-environmental control system. By using the system, the photosynthetic rate on 8th day for the Ipomoea batatas(L.)lam plantlets on stage Ⅲ in vitro was measured, and then, the photosynthetic growth model related to 2 factors of CO₂ concentration and photosynthetic photo flux density was developed.