The invention discloses a main shaft radial rotation error measuring device based on target object trajectory tracking and belongs to the technical fields of precision instrument manufacturing and measurement. The main shaft radial rotation error measuring device comprises an image acquisition device, an etalon, a clamping device, a to-be-measured main shaft, a base, a gantry support, an X-direction displacement guide rail, a Y-direction displacement guide rail and a Z-direction displacement guide rail, wherein the clamping device is used for clamping the etalon. The X-direction displacement guide rail is fixed to the base through the gantry support. The Y-direction displacement guide rail moves in the direction where the X-direction displacement guide rail is located. The Z-direction displacement guide rail moves in the direction where the Y-direction displacement guide rail is located. The image acquisition device moves in the direction where the Z-direction displacement guide rail is located. The etalon is mounted on the rotation end face of the to-be-measured main shaft through the clamping device. The etalon is of a barrel-shaped structure, wherein the top end of the barrel-shaped structure is provided with a target object, and light can be emit from the interior of the barrel-shaped structure. The target object is an eccentric hole. By means of the main shaft radial rotation error measuring device, a high-sampling-frequency capacitance transducer is not needed, so that equipment cost is lowered; and in addition, high-precision measurement of radial rotation errors of the high-rotation-speed main shaft can be achieved.