There is overlarge shearing force when shearing high-strength, large-diameter, thick drill pipes in ultra-deep well drilling by traditional shearing techniques. Excessive shearing force will result in oversized blowout preventers (BOPs) and higher control hydraulic pressure, even over the ability of the control hydraulic pressure. This article proposed a technique using external pressure differences to assist in shearing drill pipes, significantly reducing the maximum shearing force. After obtaining experimentally the Johnson–Cook constitutive model parameters for V150 drill pipe material, the present study simulated and analyzed the shearing characteristics of ram BOP when shearing the 5 7/8 in. V150 drill pipe under external pressure differences and hanging loads. The results show that the higher the strain rates of the sheared drill pipe, the greater the maximum shearing force, with temperature having little effect during shear. Before shearing instability, the greater the external pressure difference, the smaller the shearing force. When occurring shearing instability, there will appear an additional flattening and instability phase. The hanging load has little effect on the maximum shearing force but influences the instability threshold and timing. It is recommended to set the external pressure differences around 40 MPa for the 5 7/8 in. V150 drill pipe when assisting in shearing, while also need to enhance the BOP's impact resistance. The research findings provide new insights into well control operations, significantly reducing costs and offer a valuable reference for the field application of well control systems.