The seismic correlation lengths prior to the Minxian-Zhangxian earthquake occurring on July 22,2013,have been studied using the single-link cluster analysis(SLC)method.The catalogs used were downloaded from the China Earthquake Data Center(CEDC;http://data.earthquake.cn/data/).Earthquakes that occurred during the five years prior to the main shock until its origin time were located in an area centered on the main shock(34.5N°,104.11°E)with a range of 32°~37°,102°~107°.We selected 2.5(ML)as the minimum magnitude and verified the completeness of the catalogs prior to the mainshock using the Gudengberg formula,which showed good results above magnitude ML=2.5.The result shows that an increase in correlation length for moderate-sized earthquakes was observed nearly 1.5years before the main shock and can be fitted well by the power law.For the preparation and occurrence stage of the earthquake process characterized as a self-organized critical phenomenon,direct testing for the increase in spatial correlation lengths prior to large earthquakes is an independent approach used to detect critical point behavior in observed seismicity.The results of this study show that the self-organized critical point characteristic was been observed prior to the Minxian-Zhangxian MS6.6earthquake.The results from the many earthquake applied to systematically extract a region of increased correlation length from a dataset.Estimates for the epicenter of an expected mainshock may be then obtained from the results of the grid search.