The 2013 summer drought led great losses to Ningbo City. This paper used the data from the rainfall stations in Ningbo City to assess the drought and its grade with the method of continuous rainless days and method precipitation anomaly, and gave out an improved method of regression analysis. By comparison with the historical droughts in the same period, the 2013 summer drought in Ningbo City is most serious one in the past 60 years, being 30-year return with frequency analysis.