Antibiotics are widespread in various environmental media, and may pose a potential threat to aquatic ecosystems and non-target aquatic organisms. Florfenicol (FLO) is one of the most commonly used antibiotics in aquaculture, and extensively used to substitute chloramphenicol with its strong sterilization and low adverse effect. In this study, flounder Paralichthys olivaceus , an important economic fish species in seawater was used as an experimental subject. Five exposure concentrations of FLO (including environment-related concentrations) were set at 0, 0.01, 0.1, 1, and 10 mg/L. Effects of FLO exposure for 168 h on growth and development, motor behavior, antioxidant enzyme activity, malondialdehyde (MDA) content, and thyroid hormone level of P . olivaceus larvae were studied in pre-larvae (1 dpf) and post-larvae (20 dpf). The results show that the short-term FLO exposure could promote the larvae growth to some degrees, but inhibit them as the exposure time prolonged. For pre-larvae, FLO at 0.01 mg/L could stimulate the motor nerve system and increase the swimming ability, but inhibited it at 1 mg/L. With the increasing dosage of FLO, the superoxide dismutase (SOD) and MDA contents were elevated, reaching the maximum in the 1 mg/L FLO group. The pre-larvae were more sensitive than the post-larvae to FLO in the environment, and the growth and immune resistance could be damaged with long exposure. Post-larvae were more tolerant to extal pollutants, FLO at 1 mg/L could promote the motor behavior and reduce SOD and MDA contents. Therefore, FLO can be used as an antibiotic at a proper concentration but as a drug to prevent disease in a long-term way.