The invention provides an automatic assembling tool for an impeller box and a centre of a water meter movement. The automatic assembling tool is characterized by comprising a rotating plate, a centre vibration output mechanism, a centre clamping and taking mechanism, an impeller box vibration output mechanism, an impeller box clamping and taking mechanism, a clamping mechanism and an impeller box discharge mechanism. The rotating plate can rotate in a gap mode under driving of a motor. The upper end face of the rotating plate is at least provided with four installation bases for impeller boxes to cover in the circumferential direction. The middle of each installation base is provided with a vertical channel for a centre to be inserted. The impeller box discharge mechanism can unload the assembled impeller boxes from the installation bases. The centre clamping and taking mechanism, the impeller box clamping and taking mechanism, the clamping mechanism and the impeller box discharge mechanism are sequentially arranged around the rotating plate. Compared with the prior art, the automatic assembling tool has the advantages that an automatic machine structure is adopted, manual operation can be completely replaced, and therefore the assembling efficiency is greatly improved, and the quality of the assembled products is reliable and uniform.