The invention discloses a building 3D printer follow-up trowelling mechanism which comprises a follow-up rotating mechanism, an automatic scraper blade height lifting mechanism, an adjustable scraperblade and a follow-up scraper blade control system, wherein the follow-up rotating mechanism is connected to a printing extruding machine, the automatic scraper blade height lifting mechanism is connected with the follow-up rotating mechanism, the adjustable scraper blade is connected to the automatic scraper blade height lifting mechanism, and the follow-up scraper blade control system is connected with the follow-up rotating mechanism to control an angle of the adjustable scraper blade. The building 3D printer follow-up flattening mechanism disclosed by the invention can automatically followa motion curve of a printer spray head to further rotate the position of the scraper blade in real time, so that the horizontal line of a vertical scraper blade face of the scraper blade is always parallel to a motion straight line curve or parallel to the tangential direction of a curve track; thus, a function of automatically flattening printing lines in a printing process is achieved, surfacequality of building 3D printer printing members is effectively improved, a subsequent manual modifying technology is reduced, and the efficiency of the 3D printer is improved.