Multifunctional Fe3O4@TiO2-GO (FTG) magnetic composites have extensive application potential. However, the effect of TiO2 shell thickness as well as its underlying mechanism remains murky and motivates our study. Herein, FTG composites with the optimized TiO2 shell thickness were synthesized. The characterization results show that the FTG photocatalysts had the advantages of easy magnetic separation, extended light absorption range (>600 nm), and efficient charge separation properties. The photo catalytic activity of FIG was performed for photo-degradation of enrofloxacin (ENR) under visible light. The FTG composites with TiO2 layer thickness of 17 nm exhibited the highest photocatalytic activity, and significant recycle efficiency. The photocatalytic mechanism, as characterized using electrochemistry, ESR, and in-situ QXAFS analysis, indicates that this optimal TiO2 shell thickness delicately balances the generation and effective transport of electrons, and therefore is optimal for the production of hydroxyl radicals. This study provides new insights into the fabrication of TiO2-based materials and their photocatalytic application. (C) 2017 Elsevier B.V. All rights reserved.