A flatness measuring apparatus includes a measuring assembly and a movable assembly. The measuring assembly includes a measuring unit. The measuring unit includes a supporting base, a loading block, and a limiting block mounted above the supporting base. The supporting base defines a receiving hole. The loading block is removably received in the receiving hole. The loading block is coplanar with the supporting base and cooperatively defines a limiting channel. A distance between the limiting block and the supporting base is substantially the same as a maximum allowable height of a workpiece, thereby only allowing qualified workpieces to slide therethrough and catching unqualified workpieces. The movable assembly connects to the measuring assembly to drive the loading block to disengage from the receiving hole and make the unqualified workpieces drop down from the measuring unit.