Electric treatment is a promising method to improve the cold flowability of waxy crude oil by taking advantage of the electrorheological (ER) effect of the crude oil. However, the efficacy of viscosity reduction under an electric field differs significantly among various crude oils. This paper investigates the role of graphene nanoparticles (GNPs) in enhancing the ER effect of a waxy crude oil with less sensitivity to the electric field. It is found that the addition of GNPs may significantly enhance the ER effect and its durability. Quantitative results revealed that with 200 ppm GNPs, the maximum viscosity reduction reaches 78.4% (vs. 57.1% for undoped oil) at 34 °C, and the treatment time to achieve 57.1% viscosity reduction is shortened from 10 min to 2 min. Furthermore, GNPs reduce the energy density threshold for maximum viscosity reduction from 1.08 J/mL to 0.96 J/mL, weaken the structure of the gelled oil (yield stress reduction: 80.1% vs. 43.5% for undoped oil), and extend the durability of viscosity reduction from 5 to 12 h. It is further demonstrated that both the added GNPs and charged particles in crude oil accumulate on the surfaces of wax particles under an electric field, and the effect of GNPs’ nano-templating and the electrostatic repulsion of charged particles work together, weakening the wax particle interactions and thereby enhancing the ER effect. The study provides an approach to the enhancement of the cold flowability improvement efficacy by electric treatment for waxy crude oil.