An auxiliary device for a mechanical arm includes a supporting base, two reciprocating engines, a rotational plate, and four suction members. The reciprocating engine includes a cylinder and a driving rod. The cylinder is configured for driving the driving rod linearly and reciprocating along its longitudinal axis. The cylinder is rotatably fixed to the supporting base, thereby defining a first rotational axis therebetween. The driving rod is rotatably fixed to the rotational plate, thereby defining a second rotational axis therebetween. The rotational plate is also directly and rotatably fixed to the supporting base to be rotatable about a third rotational axis defined therebetween. The three rotational axes are parallel. The four suction members are fixed to the rotational plate.