The invention relates to a detecting device, in particular to a board flatness detecting device for building monitoring and aims at providing a board flatness detecting device for building monitoringso that the flatness detecting accuracy can be improved and the operation becomes convenient. The detecting device comprises a bottom plate, support rods, rubber pads, support plates, a motor, a bearing base, a lead screw, and a nut. The support rods are fixedly connected to the left part and the right part of the upper side of the bottom plate; the rubber pads are arranged at the upper sides of the support rods; the support plates are fixedly connected to the left part and the right part of the upper side of the bottom plate and are arranged at the outer sides of the support rods; and the motor is installed at the upper part of the right side of the left support plate. With a rotating rod, a push rod, a cylinder body, a piston, an air inlet pipe, an air outlet pipe, a check valve and a spray head, the operator blows a board by holding the tail of the air outlet pipe to blow away sands on the surface of the board, so that the accuracy of the board flatness measurement result is improved.