The invention discloses a building external wall concrete trowelling device. The building external wall concrete trowelling device comprises a fame, a smearing mechanism, a feeding mechanism and a leveling device, the suspended frame is perpendicular to a wall, makes contact with the wall through rolling wheels, and keeps a certain distance from the wall, concrete slurry in a material trough is conveyed and sprayed to the surface of the wall through a conveying pump, a smearing disc is driven by a motor to rotate to extrude and trowel the concrete slurry, when the frame moves downwards, the leveling device levels concrete adhering to the wall. According to the building external wall concrete trowelling device, the problems that original building external wall concrete coating and leveling efficiency is low, high-altitude operation is needed, and potential safety hazards exist are solved, and the building external wall concrete trowelling device has the characteristics that the structure is simple, the coating and leveling efficiency is high, planeness is precise, high-altitude operation of operators is not needed, and the safer effect is achieved.