The invention provides a mechanical arm structure applied to a stacking robot. The mechanical arm structure is characterized in that one end of a large arm assembly is connected with the first end ofa small arm assembly and the first end of a triangular plate through an elbow shaft, the second end of the small arm assembly is connected with the first end of a wrist disc assembly through a wrist shaft, the third end of the small arm assembly is connected with one end of an elbow connecting rod through an elbow pin, the second end of the wrist disc assembly is connected with one end of the wrist connecting rod, a motor and a wrist flange are installed at the third end of the wrist disc assembly, a mechanical clamp is installed at the bottom end of the wrist flange, the other end of the wrist connecting rod is connected with the second end of the triangular plate, the third end of the triangular plate is connected with one end of the connecting frame rod through a connecting rod pin, theother end of a connecting frame rod is connected with a waist disc of the stacking robot through a connecting frame pin, the other end of the wrist connecting rod is connected with a crank on the waist disc of the stacking robot through a rocker arm pin, and the other end of the large arm assembly is connected with a speed reducer on the waist disc of the stacking robot. The mechanical arm structure is reasonable and compact in structure, suitable for customized production of small-batch stacking robots, and convenient to popularize and apply.