The multiple-carrier model for thermal/electrical poling is presented.Under the approximation of the full depletion,the calculating formulas for the width and electrical field of depletion region are obtained.They show that the relationship for the width and the maximum electrical field in depletion region against the applied voltage is square-root,while the conversion efficiency of second-harmonic generation is square with voltage.The width of the depletion region is 3～8 μm,and the maximum electrical field below the anode surface 1～6 μm is 1～4 V/nm.