In the present paper, OH radicals generated by pulsed corona discharge in humidified air, N2 and Ar in a needle-plate reactor were measured by emission spectra. With the analysis of the emission spectra, the influence of pulse peak voltage and frequency on OH radical generation was investigated in the three kinds of background gases. The influence of the gas humidity on the generation and the distribution of OH radicals in the electric field was also discussed in detail. The authors studied the influence of the gas humidity on the generation of OH radicals in the electric field by the control of accurate change in humidity, and we also studied the distribution of OH radicals in the electric field in different background gases including humidified air, N2 and Ar by the accurate change in scales. The experiment shows that the output of OH radicals grows as the pulse peak voltage and frequency grow, but the influence of gas humidity on the process of generating OH radicals by pulsed corona discharge depends on the discharge background. The rules of the generation change when the background gases change. As the humidity in the background gases grows, the amount of OH radicals grows in the air, but it grows at first and decreases at last in N2, while it decreases at first and grows at last in Ar. The distribution of OH radical shows a trend of decreasing from the needle-electrode to its circumambience.