Photosynthetic inhibitory herbicides widely used to control weeds, such as triazine herbicides, will have toxic effects on the photosynthesis of microalgae once entering aquatic environment, thus posing serious threats to the ecological balance of aquatic systems. However, at present, in terms of toxicity detection of triazine herbicides to microalgal photosynthesis, the commonly used photosynthetic fluorescence parameters such as the maximum photochemical quantum yield of photosystem II (F_V/F_M) and performance index (PI_ABS) are not sensitive enough in response to the toxicity of triazine herbicides. In order to seek for a suitable response index which can be used to rapidly and sensitively predict the toxicity of triazine herbicides to the photosynthesis of microalgae, in this study, a common freshwater microalgae Chlorella pyrenoidosa was used as the test organism, and the impacts of four triazine herbicides atrazine, terbuthylazine, propazine and simazine on the chlorophyll fluorescence rise kinetics (OJIP) curve of C. pyrenoidosa were first investigated. On this basis, a novel photosynthetic response index (PI) for detecting the toxicity of triazine herbicides was successfully constructed according to the change characteristics of OJIP curve of exposed C. pyrenoidosa, then its response performance to triazine herbicide toxicity was further verified. The results indicate that the significant changes in OJIP curves of exposed C. pyrenoidosa compared with the control were mainly caused by the increases of fluorescence intensity of O-step (F_o) and variable fluorescence intensity between J-step and O-step (Fv_(J-O)) and the reductions of variable fluorescence intensities between I-step and J-step (Fv_(I-J)) and between P-step and I-step (Fv_(P-I)) of OJIP curve. The constructed PI with F_o, Fv_(O-J), Fv_(J-I), Fv_(I-P) of OJIP curve as variables showed very good Logistic curve concentration-response relationships with each triazine herbicide. By verification, the response sensitivity of PI to each triazine herbicide were all superior to that of F_V/F_M and PI_ABS, and PI showed good agreement with PI_ABS in predicting the toxicity ranking of different triazine herbicides to C. pyrenoidosa photosynthesis. Therefore, the constructed PI was a more suitable and reliable response index for sensitively detecting the toxicity of triazine herbicides to microalgae photosynthesis. This study provides a new and optional index for sensitive determination of the toxicity of triazine herbicides in water based on chlorophyll fluorescence induction kinetics technology.