This mechanical arm is designed aiming at that the online loading of the products in bags mainly relies on manpower, and the problems of that the labor intensity of the workers is large, the loading efficiency is low and the environment is bad, and etc. First of all, the overall structure, main functions and operating principles of this mechanical arm were given. Second, doubledeck falling bag mechanism with four buffer spring, swing mechanism which could make up for the distance tolerance between the blanking mechanism and the baffle plates of the freight car, and width and narrow change mechanism of the blanking door were designed emphatically, which would reduce the falling impact force of the products in bags and achieve the maximum utilization of the space of the carriage to adapt the products in bags with different specifications. Lastly, stress analysis for the spring and key shaft has been carried out, to certify the rationality of the structural design from the aspects of practicability and reliability.