Abstract: This paper proposes a new method to calculate the single particle displacement damage currents in ultra-low leakage current diode. The spatial distribution of primary knock-on atoms in the diode is simulated with SRIM. Based on the SRIM results, the SPDD current steps in the diode irradiated by 252Cf are calculated with Shockley-Read-Hall theory. The theoretical calculations of single particle displacement damage currents conduce to good agreements with experimental data. A stratified constant-gradient method is proposed to approximately describe the non-uniform distribution of electric field in the depletion region, therefore, the contributions of defects created in different positions in the depletion region to the increased leakage current can be calculated independently. The results show that the carriers emission is most significantly enhanced by electric field near the PN junction. Compared to the calculated results without considering the field-enhanced emission effect, the contribution of single defect near the PN junction to the increase of leakage current is two orders of magnitude higher when the electric field-enhanced emission effect is taken into consideration. Besides, the SPDD currents induced by 80 MeV Nd ions are generally higher than that by 106 MeV Cd ions. The SPDD currents induced by these ions are mainly between 1 fA to 1 pA.