Catalytic aquathermolysis in situ upgrading and reducing the viscosity of heavy oil in the reservoir remarkably enhances the recovery and is considered as a promising technology. However, the low catalytic efficiency and inferior dispersity in both water and oil limit its applications. In the present work, spherical polymer brush nanocatalysts were synthesized, in which nano-TiO2 is the core and poly(vinyl imidazole) (PVI)-loading nickel cations are polymer brushes. The chemical characteristics, polymer-grafting content, nickel-loading content, and morphology of as-prepared catalysts were characterized by infrared (IR) spectroscopy, thermogravimetric analysis, inductively coupled plasma optical emission spectrometry, scanning electron microscopy, and transmission electron microscopy. The polymerization degree of PVI was analyzed by proton nuclear magnetic resonance (H-1 NMR) spectra. The effects of the nickel -loading content, catalytic conditions, and hydrogen donor on the viscosity of heavy oil were studied. The results show that the heavy oil is catalytically cracked by the synthesized catalysts, which leads to the reduction of oil viscosity. The viscosity reduction is enhanced by the increase of the nickel-loading content, catalytic temperature, dosage of catalyst, and hydrogen donor. The rheological behaviors in terms of flow curve, thixotropy, viscoelasticity, and time dependence of cracked oil were studied. To explore the cracking mechanism, the four compositions of heavy oil before and after aquathermolysis were compared. The extracted asphaltenes and resins were further-analyzed by elemental analysis, H-1 NMR spectra, and IR spectroscopy. The organic compounds in reacted water were characterized by gas chromatography-mass spectrometry. It is found that the content of light saturates is much increased after aquathermolysis, along with the distinct decrease of resins. From the structure change of resins, such as the decrease of hydrogen/carbon and methylene/methyl ratios and increase of aromaticity and aromaticity condensation, the increased light saturates are due to the dissociation of alkyl side chains in resins. In addition, the aromaticity and aromaticity condensation in asphaltenes are found decreased, which is because of the fragmentation and depolymerization of large aromatics. Meanwhile, the loss of oxygen in both asphaltenes and resins is connected with the phenols found in the reacted water, indicating the breakage of the C-O bond and heteroaromatic ring-open reaction in both asphaltenes and resins during aquathermolysis.