In order to effectively clean the residue of propellant inside different caliber gun barrels, a gun barrel cleaning robot with an adjustable variable diameter mobile mechanism is designed, and the mechanical performance analysis of the forces acting on it in the pipeline is conducted. Based on the required functional requirements and performance indicators, the working principle and structural scheme of the robot are determined. The driving mechanism, walking mechanism, and cleaning brush of the robot are designed. The mechanical modeling of the forces acting on the robot in the pipeline is carried out based on the D’Alembert’s principle. The resistance of the cleaning brush, anti-slip 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 stably operate within the range of 104mm to 155mm caliber unrifled gun barrels, and the design scheme is feasible.