Powdery photocatalyst has long been studied, yet its low reusability in suspension system is the bottleneck which hinders its large-scale application. An alternative method to overcome this drawback is developing magnetic recyclable composite. In this work, we prepared lanthanum-nitrogen co-doped titania/strontium ferrite/diatomite (LN-TSD) ternary composite via sol-gel technology. The as-prepared materials were characterized and analyzed, and the photocatalytic activity was evaluated via the elimination of antibiotic oxytetracycline (OTC) under visible light illumination. Charge transfer happened at the heterojunction interface between TiO2 and strontium ferrite, and thus improved the photocatalytic efficiency by inhibiting the recombination of charge carriers. The photodegradation rate of OTC was accelerated by the high adsorption ability of substrate diatomite, due to the adsorption and degradation synergistic effect between photocatalytic components and substrate diatomite. The optimal dopant amount and catalyst dosage were determined, and the corresponding LN-TSD sample showed its degradation rate of OTC as 95.5% after 2.5 h. The ternary LN-TSD composite could simply be separated from OTC solution via an external magnet, and the repetition tests indicated that, after 5 turns of repetition, only slightly decrease of degradation rate could be observed (dropped to 94.2%). This composite was promising to be applied in wastewater remediation process, due to its good photocatalytic activity under visible light, as well as its good reusability and stability.