Pharmaceutical wastewater with a high content of antibiotic residuals can greatly inhibit the activity of microorganics. Dynamic monitoring of the dissolved antibiotic content is desirable to maintain optimal biodegradation conditions. For this purpose, an on-line analysis method for sulfonamides in pharmaceutical wastewater was developed by using magnetic molecularly imprinted polymers (MMIPs) extraction and near infrared spectroscopy. An auto-sampling device was designed to perform the extraction procedure. The MMIPs synthesized had high affinity and selectivity to target sulfonamides. The automatic extraction process, including condition, extraction, washing and elution, could be carried out in several minutes, resulting in the analyte transferring from complex matrix (wastewater) to acetonitrile and thus being concentrated. The recoveries achieved ranged from 76.3% to 86.8% with relative standard deviations of 3.6% to 7.9%. Partial least squares regression was used for modeling and the proposed model predicted R² = 0.977 for the HPLC method and the absolute residuals of prediction were less than 5 mg L⁻¹. This method can rapidly provide a reference for the drug content and is promising for on-line analysis of sulfonamides in pharmaceutical wastewater. By changing the template molecule of MMIPs, it can also be used for the determination of other antibiotics.