Abstract: We first expounded the derivation process of positive corona criterion for rod-plate gap in detail, and established a photoionization model for calculating the positive and negative DC corona onset voltage of rod-plate gap by using positive and negative corona criterion. Values of collision ionization attachment coefficients, photon absorption coefficient, emission coefficient of photon surface and other parameters in the model are determined. The positive and negative DC corona discharge test of the rod-plane gaps at 10 cm, 12 cm, 14 cm and 16 cm was carried out. The positive and negative DC onset voltage characteristics under different gap distances were obtained by the UV imager and the high potential current acquisition system. The calculated results were compared with the results of UV detection. The relative error is-0.8%～5.6%. Compared with the high potential current method, the relative error is-1.8%～2.5%. The calculated results agree well with the experimental results. The proposed calculation model for positive and negative DC corona onset voltage has guiding significance and important reference value for enhancing corona control in UHV transmission engineering.