In order to effectively clean the residual propellant inside gun barrels with different diameters, a gun barrel cleaning robot with an adjustable variable-diameter moving mechanism is designed, and the mechanical performance analysis of its force inside the pipeline is conducted. Based on the required functional requirements and performance indicators, the working principle and control scheme of the robot are determined. The driving mechanism, walking mechanism, and cleaning brush of the robot are designed. Based on the D"Alembert"s principle, a mechanical model is established to analyze the forces exerted on the robot within the pipeline. The resistance of the cleaning brush, anti-skid and tensioning of the walking mechanism, and driving force of the robot are analyzed through simulation. The simulation results show that the gun barrel cleaning robot can operate stably within the range of 104mm to 155mm diameter smoothbore gun barrels, and the design scheme is feasible.