The invention discloses a building surface spray painting robot. The building surface spray painting robot comprises a spraying gun, a slurry spraying machine, a large scraper blade, a traveling trolley and a control system, wherein the spraying gun is connected with the slurry spraying machine through a hose; a supporting arm is arranged at the front end of the traveling trolley; the large scraper blade is vertically and slidably mounted on the supporting arm through a guiding and sliding assembly; the traveling trolley is provided with a lifting mechanism which is used for driving the large scraper blade to slide vertically; the spraying gun is transversely and slidably mounted on the large scraper blade; a spraying head of the spraying gun and the large scraper blade are arranged in the same direction; the large scraper blade or the guiding and sliding assembly is provided with a driving mechanism which drives the spraying gun to slide transversely; the slurry spraying machine is mounted on the traveling trolley; and the slurry spraying machine, the lifting mechanism and the driving mechanism are respectively connected with the control system electrically. The spraying gun is transversely and slidably mounted on the large scraper blade, the large scraper blade is slidably mounted on the supporting arm through the guiding and sliding assembly, so that automatic slurry spraying and automatic floating are realized, automation of plastering of the surface of a wall of a building is realized, and labor intensity of workers is greatly reduced.