Understanding the synergy between metal and zeolite for catalytic aquathermolysis harbours tremendous scientific and industrial importance. Herein, HZSM-5 is used as a model zeolite due to uniform physico-chemical structure, which is required to elucidate the intrinsic structure-performance relationship. We first devise a novel and scalable strategy to synthesize recyclable magnetic Fe3O4/HZSM-5 catalyst, which shows enhanced catalytic aquathermolysis performance compared to pure HZSM-5 catalyst. Moreover, the underlying synergy effect is then systematically elucidated by multi-techniques such as H-1 NMR, HRTEM, Py-IR, NH3-TPD, SARA and EL. Importantly, it is found that the change of acidity is not the dominating reason for enhanced performance. The synergy not only favors dispersion of Fe3O4 nanoparticles on zeolite, but also effectively breaks C-S bond and reduces the percentages of resin and asphaltene. Furthermore, the designed Fe3O4/zeolite catalyst effectively reduces the viscosity of heavy crude oil by 85.0%. This work sheds new light on the design of highly efficient heterogeneous catalysts for catalytic aquathermolysis.