Acute toxicity (EC₅₀₋₃₀ min) and chronic toxicity (EC₅₀₋₂₄ h) of 21 antibiotics on Photobacterium phosphoreum was observed and Quantitative structure-activity relationship (QSAR) models were developed, respectively. By comparing these two QSAR models, the following model was established, log(1/EC₅₀₋₂₄ h) = 1.8283 + 1.1503log(1/EC ₅₀₋₃₀ min) + 0.2872Elomo - 0.0901DM + 0.0003PMIZ + 0.0088PSA - 0.0382SD, r² = 0.8513, where Elomo is lowest occupied molecular energy, DM is dipole moment, PMIZ is principal moment of inertia Z, PSA is polar surface area, and SD is sum of degrees. It provides a good way for us to obtain EC₅₀₋₂₄ h from EC₅₀₋₃₀ min, because the later is far easier to observe than the former.