Yunnan Province is in southwest China, where the seismicity has been active since ancient times. Generally, the uncertainty of historical earthquake parameters is larger. To amend the parameters of historical earthquakes, we have developed a new intensity attenuation equation. From 2000 to 2018, there were 25 instrumentally recorded earthquakes with 5.0–6.6 in Yunnan. The parameters of those earthquake events, including their epicentral locations and magnitudes, are determined with high accuracy. Meanwhile, total intensity values of 1345 intensity data points have been carefully assessed by survey. With both accurate earthquake parameters and valuable intensity data, a new intensity attenuation equation has been established. The result shows the optimal intensity magnitude can be calculated from the mean of , in which is the distance between the epicenter and the surveyed seismic point. By adapting the method proposed by Bakun and Wentworth (1997) for determining earthquake source parameters directly from historical intensity data, we have tested retrospectively the new attenuation on the 25 instrumentally recorded earthquakes. Then this attenuation was applied to deal with the parameters of two historical earthquakes, the 26 February 1713 Xundian earthquake and the 11 May 1909 Huaning earthquakes. Our results reduced the uncertainty of previously estimated parameters, which were large. The amended parameters will be valuable for seismic hazard analysis and earthquake disaster reduction.