A crankshaft balancing mechanism assembling device comprises a base, a side plate fixedly connected with the base, and a W-shaped block doing linear circulating reciprocation under the driving of a power device, wherein a positioning mechanism for positioning a crankshaft balancing mechanism is arranged on the side plate; a gear positioning plate is further fixedly connected to the side plate; teeth are arranged on the gear positioning plate; two V-shaped surfaces are arranged on the W-shaped block; the included angle between the V-shaped surfaces is the same as that between two process assembling surfaces of a balancing block; during assembly of the crankshaft balancing mechanism, the positioning mechanism on the side plate is used for positioning a balancing seat assembly in the crankshaft balancing mechanism; under driving of the power device, the W-shaped block utilizes the two V-shaped surfaces to tightly clamp the balancing block of the crankshaft balancing mechanism; the teeth of the gear positioning plate are meshed with a gear of the crankshaft balancing mechanism, so that a guiding positioning role on the assembling of the gear is played. The device has the advantages of being smart in structure, easy to process, high in practicability and convenient to popularize on the premise of ensure that the crankshaft balancing mechanism is assembled correctly.