A system for detecting an object in three-dimensional space includes: four emitters defining a rectangular detection area, the emitters being disposed around the four vertices of the rectangular detection area respectively; two receivers being disposed at the midpoints of two edges of the rectangular detection area respectively; and an electronic controller being connected with the emitters and the receivers and configured to control the emitters to radiate light in predetermined wavelengths, to control the receivers to capture light in the predetermined wavelengths reflected by the object and thereby output a plurality of signals, and to convert the signals into coordinates of the object.