The disclosure provides a millimeter-wave/terahertz-wave imaging apparatus and a method for detecting a human body or an article. The millimeter-wave/terahertz-wave imaging apparatus comprises a quasi-optical assembly, a detector array, and a reflecting plate adjusting device. The quasi-optical assembly is used for reflecting a beam that is irradiated spontaneously or reflected by a detected object and converging the beam to the detector array and includes a plurality of reflecting plates for receiving and reflecting the beams from the detected object; the angles between the plurality of reflecting plates and the normal of the view field at which the detected object is located are different; and the angles between the plurality of reflecting plates and the horizontal plane are different. The detector array is used for receiving a beam from the quasi-optical assembly. The reflecting plate adjusting device includes a rotating mechanism for driving the plurality of reflecting plates to rotate in a horizontal direction and thus the plurality of reflecting plates can reflect beams that are irradiated spontaneously or reflected by parts, arranged at different vertical positions of the view field, of the detected object, thereby realizing comprehensive reflection of the view field. Sampling points are concentrated in the middle of the field of view; the sampling points are distributeduniformly in most of areas of the view field; and the interpolation becomes convenient.