The invention discloses a building steel bar surface rust removing device which comprises a base. An electric cabinet is arranged at the left end of the upper side of the base. Supporting columns are arranged at the four corners of the upper side of the base. A supporting base is arranged on the upper sides of the supporting columns. A box body is arranged on the upper side of the supporting base. First motors are arranged at the upper end and the lower end of the left side of the box body. Lead screws are arranged on the right sides of the first motors. Internal threaded sleeves are arranged on the lead screws. First telescopic rods are arranged on the inner sides of the internal threaded sleeves. A clamping ring is arranged on the lower side of each first telescopic rod. A fixed ring is arranged in the middle of the interior of the box body. An inner gear ring is arranged in the fixed ring. A driving gear is arranged at the lower end of the interior of the inner gear ring. A second telescopic rod is arranged on the inner side of the inner gear ring. A friction ring is arranged on the inner side of the second telescopic rod. The middle portion of the right side of the fixed ring is provided with a connecting cavity. A dust discharging pipe is arranged at the right end of the lower side of the connecting cavity. A dust collecting cavity is arranged on the lower side of the dust discharging pipe. According to the building steel bar surface rust removing device, the surfaces of steel bars can be automatically polished effectively and conveniently, dust generated in the polishing process can be effectively removed, and the ambient environment is protected.