Abstract: The gas switch is one of the key components in pulsed power system and the impendence of spark channel greatly affects the efficiency of power delivery and the rising time of voltage applied to the load, but accurate measurement of spark resistance under oscillatory damped current pulse is difficult. In this paper, a spectrographic method is put forward. Through measuring the emission spectrum of spark channel with time and spatial resolution, the spark radius and conductivity are estimated and the spark resistance is calculated. The results show that electron temperature of nitrogen spark is about 2-3 eV while the conductivity of spark channel increases rapidly at first and then gradually decreases with a maximum of 16 000 S. As the spark channel develops, spark resistance drops rapidly from insulation and reaches a saturation value, and the time-dependent property of spark resistance fits well with the calculated results. In the pressure ranged from 3 to 100 kPa, the spark radius decreases and the spark resistance increases with the increase of pressure.