Abstract: The investigation on the linear characteristics of the PIN detector with calculation and experiments was presented. The maximum linear current of the PIN detector was measured with a fast pulse X-ray generator and a pulse xenon optical source. The maximum linear current is directly proportional to the magnitude of the applied bias voltage to the PIN detector. The maximum linear current measured by penetrating radiation is larger about 20% than that by non-penetrating radiation. It is useful that the maximum linear current of the PIN detector is measured by pulse xenon optical source.