A robot arm assembly includes a first mechanical arm, a second mechanical arm, a third mechanical arm, a first transmission assembly and a second transmission assembly. The first mechanical arm, the second mechanical arm and the third mechanical arm are rotatably connected together in that order, and are respectively configured to rotate along a first axis A, a second axis B and a third axis C. The first transmission assembly is assembled within the first mechanical arm and the second mechanical arm, and is further coupled with the third mechanical arm. The second transmission assembly is assembled within the first mechanical arm and sleeved on the first transmission assembly, the second transmission assembly is further coupled with the second mechanical arm.