1. An object position determining device, comprising: a light emitting module; a first reflecting unit, being a long strip; a second reflecting unit, being a long strip; a mixed-type reflecting unit, comprising a stripped L-shaped reflecting mirror and retroreflector, said retroreflector being positioned above or below said reflecting mirror, said mixed-type reflecting unit, said first reflecting unit and said second reflecting unit being surrounded to form an approximately rectangular work area, said first reflecting unit and said second reflecting unit being respectively positioned at two side ends of said mixed-type reflecting unit, a contact of said first reflecting unit and said second reflecting unit being a corner of said work area set to be a reference point, a light beam of said light emitting module being emitted from said reference point illuminating said whole work area or scanning said whole work area; a light detecting unit, detecting said light beam blocked directly and indirectly through said reflecting mirror to obtain a first signal, and detecting said light beam blocked by said object through said retroreflector to obtain a second signal, wherein said first signal comprises a plurality of first weak signals corresponding to said object, and said second signal comprises at least one second weak signal corresponding to said object; a signal processing unit, electrically connected to said light detecting unit, said signal processing unit processing said first signal and said second signal to obtain a plurality of included angles A 1 corresponding to said plurality of first weak signals and a plurality of included angles A 2 corresponding to said plurality of second weak signals formed by respectively intercepting said light beam emitted from said reference point and blocked directly and indirectly by said object; a microprocessor, electrically connected to said signal processing unit, said microprocessor combining said plurality of included angles A 1 with each said included angle A 2 , respectively converting said combinations to obtain a plurality of coordinates, then comparing said plurality of coordinates with one another, and selecting at least two of said coordinates with the same value among said plurality of coordinates, thereby confirming a relative coordinates of said objects on said work area.