The invention relates to an automatic painting device for the inner wall of a round pit, and solves the problems that in the prior art, concrete is difficult to smear on the inner wall of the round pit, and the smearing efficiency is low. According to the technical scheme, the automatic painting device comprises a concrete smearing device, a storage barrel and a driving smearing mechanism, wherein the driving smearing mechanism comprises a vertical moving unit, a circular motion unit and a reciprocating rotation unit, the concrete smearing device is rotationally connected with the storage barrel, the circular motion unit drives the concrete smearing device to do circular motion, and after at least one circle of circular motion is performed at the same horizontal position, the vertical moving unit drives the concrete smearing device to move downwards; and when the concrete smearing device and the protruding end are located at the same horizontal position, the reciprocating rotating unit drives the concrete smearing device to rotate, and when the concrete smearing device makes contact with the protruding end, the reciprocating rotating unit drives the concrete smearing device to rotate reversely. The automatic painting device is novel in conception, ingenious in structure and high in practicability.