A passive security checking device, wherein the passive security checking device comprises: an optical means configured for reflecting and converging millimeter wave/terahertz waves spontaneously radiated by an object under check and millimeter wave/terahertz waves reflected by background environment; and a detector array configured for receiving converged millimeter waves/terahertz waves and converting the received millimeter waves/terahertz waves into electrical signals; wherein the detector array comprises a receiving antenna unit, wherein the receiving antenna unit comprises a plurality of horn antennas each comprising a horn body and a waveguide connected to the horn body and a ratio of a long side to a short side of an horn opening of the horn body is greater than 1.2; wherein a polarization direction of each of the plurality of the horn antennas is horizontal; wherein the detector array comprises a plurality of detectors, each detector is connected to one of the plurality of the horn antennas, and for each detector and its respective horn antenna, the detector comprises a normal temperature direct detector comprising a coplanar line and a transition waveguide for inputting millimeter waves/terahertz waves, the polarization direction of the waveguide of the horn antenna being configured to be horizontal polarization, the polarization direction of a waveguide port surface of the waveguide of the horn antenna matching the polarization direction of a waveguide port surface of the transition waveguide; and/or a polarization direction from the transition waveguide to the coplanar line matches the polarization direction of the waveguide of the horn antenna; wherein the optical means comprises a grid-type reflecting plate, and a distribution of grids on the grid-type reflecting plate matches the polarization direction of each of the plurality of the horn antennas in the detector array. an optical means configured for reflecting and converging millimeter wave/terahertz waves spontaneously radiated by an object under check and millimeter wave/terahertz waves reflected by background environment; and a detector array configured for receiving converged millimeter waves/terahertz waves and converting the received millimeter waves/terahertz waves into electrical signals; wherein the detector array comprises a receiving antenna unit, wherein the receiving antenna unit comprises a plurality of horn antennas each comprising a horn body and a waveguide connected to the horn body and a ratio of a long side to a short side of an horn opening of the horn body is greater than 1.2; wherein a polarization direction of each of the plurality of the horn antennas is horizontal; wherein the detector array comprises a plurality of detectors, each detector is connected to one of the plurality of the horn antennas, and for each detector and its respective horn antenna, the detector comprises a normal temperature direct detector comprising a coplanar line and a transition waveguide for inputting millimeter waves/terahertz waves, the polarization direction of the waveguide of the horn antenna being configured to be horizontal polarization, the polarization direction of a waveguide port surface of the waveguide of the horn antenna matching the polarization direction of a waveguide port surface of the transition waveguide; and/or a polarization direction from the transition waveguide to the coplanar line matches the polarization direction of the waveguide of the horn antenna; wherein the optical means comprises a grid-type reflecting plate, and a distribution of grids on the grid-type reflecting plate matches the polarization direction of each of the plurality of the horn antennas in the detector array.