A photo-ionization detector for detecting gas concentration and method thereof. Said detector includes a first bias electrode and a first measurement electrode. The first bias electrode is connected with a first bias circuit for absorbing one type of charge ion. The first measurement electrode is connected with a measurement circuit for absorbing one type of reverse charge ion. The constituent of inert gases in said ultraviolet lamp, the material of optical window and the material of low potential electrode, which is the material of said first measurement electrode or said first bias electrode, are selected that the ultraviolet light created by the ultraviolet lamp and transmitted through said optical window incident upon the surface of said low potential electrode can create the photoelectric effect and that said driver electrodes can apply an high voltage AC signal on said ultraviolet lamp, which is high enough so that the number of discharged electrons overflow from the surface of said low potential electrode are enough to neutralize the positive ions, which are deposited near the three-dimensional space range of said low potential electrode, so that the saturated phenomenon of concentration measurement caused by the deposition of positive ions can be eliminated.