Abstract: In order to explore the viscosity reduction characteristics and mechanism of heavy oil emulsion by water bath and microwave heating, the comparative viscosity-reducing experiments by water bath and microwave heating were designed with the Venezuela heavy oil emulsion with water content of 50% and nanocomposite viscosity reducer, including the viscosity reduction law analysis, the four-fraction (saturates, aromatics, resins and asphaltenes, known as SARA) separation analysis, the full two-dimensional gas chromatography-mass spectrometry (GC-MS) analysis, the Fourier transform infrared spectroscopy (FT-IR) analysis and the viscosity rebound experiment. The rules for effect of viscosity reducer mass fraction, heating temperature and heating time on the apparent viscosity of heavy oil emulsion were determined. The experimental results show that: The mass fraction of viscosity reducer has the greatest influence on the apparent viscosity. The microwave can affect the polar components and water molecules of crude oil and its non-thermal effect can further reduce the apparent viscosity of heavy oil emulsion. The light components can be obtained from the colloid and asphaltene lamellar molecules with a thick cyclic aromatic nucleus, polycyclic or heterocyclic isomeric alkyl by microwave heating and the mass fraction of heteroatom group can be reduced. The apparent viscosity of oil sample heated by microwave increased slightly within 30 days, but the viscosity reduction rate was still kept at 87.44%. Thus, these research results are very important for the viscosity reduction, production and transportation technology of heavy oil by microwave heatings.