Based on statistical analysis of thunderstorm days data recorded by 13 ground weather observation posts of Jiangsu Province from 1961 to 2008, the results show that average annual thunderstorm days decline with years, monthly variation of thunderstorm days presents a single peak, and the thunderstorms are the most frequent in July. The differences in regional distribution of average annual thunderstorm days are obvious, more in the middle while fewer in the north and south. Variation of average annual thunderstorm days also had their own characteristics. Spatial distribution of seasonal thunderstorm days changes with time. It is shown in the principal component analysis that the annual thunderstorm days of 13 cities changed at the same time, but those cities in the north of Huaihe River tended to the opposite phase. The monthly probability of occurrence of thunderstorm days changes in the same manner as the average monthly thunderstorm days, the highest probability of occurrence of thunderstorm days appeared in July. However, the probability did not seem to be exactly the same among all the cities, the probability of July is only a little higher than that of August in Suzhou and Wuxi, while the probability of cities along and north of the Yangtze River is more concentrated in July. The results of this paper can provide a basis for lightning protection and early warning.