As a clean and efficient means to reduce the viscosity of heavy oil, microwave has broad research prospects in the field of heavy oil upgrading. For the microwave-assisted heavy oil upgrading process, the catalyst research has not made a breakthrough for a long time. In this study, a one-step method was used to load copper on biochar (BC) to prepare a catalyst (Cu/BC) in microwave, which can be used as a catalyst and microwave absorbing medium in the microwave-assisted upgrading process of heavy oil. The effect of the catalyst on the upgrading of heavy oil under different microwave reaction conditions was studied. The results show that the catalyst prepared by impregnation concentration of 0.15M metal salt has the best catalytic upgrading effect on heavy oil at a microwave temperature of 260 °C for 20 min, and the viscosity reduction rate of heavy oil reaches 78.55%. Under the action of the functional groups on the surface of the catalyst and the metal, the long-chain macromolecular hydrocarbons in the heavy oil were cracked into small molecular hydrocarbons, resulting in a decrease in the viscosity of the heavy oil. The analysis results show that the catalyst also has a positive effect on promoting the breaking of C–S bonds in heavy oil. The application of metal-supported biochar catalyst provides a new idea for the study of the viscosity reduction reaction of heavy oil under microwave irradiation.