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This study developed a comprehensive characterization method for the combined degradation effect of modified fluoroquinolones (FQs) photodegradation and microbial degradation. A combination of revised 3D-QSAR model, molecular docking, path simulation inference, pharmacokinetics, molecular dynamics (MD) simulation and toxicokinetics simulation was used to construct a systematic environment-friendly drug screening system. Five derivatives were screened with significantly improved combined degradation effect (over 20%) and functional characteristics and human health parameters through combined model verification, functional and human health risk assessment. The simulation path of photo- and microbial-degradation of gatifloxacin and new gatifloxacin molecules was derived, and the reaction energy barrier was also calculated. The ratio of the total rate-determining steps change rate of the decreased energy barrier (14.10%:26.30%) was consistent with the ratio of the increased degradation performance predicted by the model (22.87%:19.77%), demonstrating the reliability of revised 3D-QSAR model and it could be applied in molecular modification. MD and toxicokinetics simulation were used to predict the binding energy and aquatic toxicity between photo- and microbial-degradation products and the degradation enzymes, which further to screen the degradation pathways with low potential environmental risks. The findings will be helpful to screen environment-friendly drug and develop appropriate strategies for its risk management.