A detector with a wide detecting range having a module of electrical circuit boards including an infrared radiation detector element; a stacked-up multifaceted arc lens assembly installed in front of the detector element which is located at the focus of the arc lens assembly to receive the focused infrared rays; and a single deflector plate conjoined to the detector element and mounted on the electrical circuit board, the signal deflector plate having symmetrically constructed reflecting units. Each reflecting unit is composed of a least two reflection planes. The invention is capable of covering detection blind zone exceeding 60 degrees both in right and left directions of the central axis of the detector element, thereby achieving a wide detecting range of over 200 degrees. A method of extending the detecting range is also taught.