A back rocker arm device of a motorcycle is arranged on a transmission box of a motorcycle engine in order to be connected with a back wheel of a motorcycle, so that a back wheel of a motorcycle can swing relative to a transmission box. The back rocker arm device comprises a first rocker arm unit which is coaxial with the transmission shaft of the transmission box and a second rocker arm unit which is coaxial with the transmission shaft. The first rocker arm unit comprises a shaft sleeve arranged on the transmission box and a first suspension arm pivoted on the shaft sleeve. The second rockerarm unit comprises a shaft rod arranged at the corresponding side of the transmission box and a second suspension arm sleeved on the shaft rod. Because the first suspension arm and the second suspension arm can be coaxially arranged on the corresponding side of the transmission box around the central axes, extra supporting labor is not needed in the assembly process, the assembly speed is improved, labor is saved effectively and the assembly security is improved.