The invention discloses a building rebar cutting device. Sliding grooves are formed in the middle portions of the upper end face of a sliding supporting base, a rebar right end fixing device is arranged on the right side of the upper end face of the sliding supporting base, and a rebar straightening device and a cutting device are arranged in the sliding grooves in a left-and-right sliding manner;a control module is arranged on the front-upper portion of the left end face of the sliding supporting base, a first rebar length scale is arranged on the portion, located on the front side of the sliding grooves, of the upper end face of the sliding supporting base, and a first identification camera is fixed to the front side of the rebar strengthening device; and the first identification camerais located over the first rebar length scale, a second rebar length scale is fixed to the rear side of the rebar strengthening device, a second identification camera is fixed to the rear side of thecutting device, and the second identification camera is located over the second rebar length scale. According to the building rebar cutting device, the overall length of rebars and the length of cut rebar segments are arranged in advance, full-automatic strengthening cutting is conducted on building rebars, manual intervention is not needed in the process, the structure is simple, and cutting is accurate.