The invention relates to a weight loss simulation device for ground testing of a satellite antenna expanding mechanism, particularly to a gravity balancing mechanical arm of a satellite antenna expanding mechanism of a multi-dimensional structure. The mechanical arm comprises an installation base, an antenna expanding following arm, a direction following assembly, an encoder, a Hooke's joint, an expansion driving motor, constant force adjustment devices, an X-axis suspension tool, a suspension point three-dimensional adjustment device, and an antenna suspension tool. The Hooke's joint is installed under the antenna expanding following arm and the direction following assembly. The encoder is installed at the hinge of the Hooke's joint. The constant force adjustment devices are installed under the Hooke's joint through suspension shafts and are connected with the X-axis suspension tool and the antenna suspension tool respectively by wire ropes. The X-axis suspension tool and the antenna suspension tool are used for connection with the X axis of the antenna expanding mechanism and an antenna respectively. The mechanical arm has many degrees of freedom and a reliable structure, is precise in pose following and force control, is safe in operation, and satisfies the need of weight loss simulation in ground testing of a satellite antenna expanding mechanism of a multi-dimensional structure.