Abstract: CIRAS-3 portable photosynthetic apparatus, as a precision instrument for measuring plant photosynthesis, plays an important role in the study of plant photosynthesis, transpiration, respiration and chlorophyll fluorescence. However, in the process of use, due to many precautions and improper operation, the experimental data error is large. In order to help the beginners to master the method of measuring CO₂ response curve of plants quickly using a CIRAS-3 photosynthesizer, and obtain more accurate test data, according to our experience, the operation steps and precautions of the measurement of CO₂ response curve were described in detail. In order to provide a certain reference for the beginners, so as to improve the efficiency of the photosynthesizer.