The invention relates to a system and a method for precisely controlling carbon dioxide concentration in a greenhouse; the system comprises a data acquisition module which acquires carbon dioxide concentration and light intensity in the greenhouse in real time, an acquisition processor which preliminarily processes the acquired data, and a control processor which manages the ranges of the light intensity and the carbon dioxide required by photosynthesis of different crops in different growth stages; the control processor judges whether a real-time acquisition value of the light intensity is within the range of the light intensity; the difference between the real-time acquisition value of the carbon dioxide and the reasonable range of the current stage is calculated; through an output control signal, a driving circuit controls an electromagnetic valve of a carbon dioxide gas tank, so that the precise control over the carbon dioxide concentration is realized; and the invention has the functions of light intensity sensing, stage control, real-time monitoring, wireless transmission, automatic control and the like, and has the advantages of low cost, simplicity in operation and wide scope of application.