Based on the time-space epidemic-type aftershock sequence (ETAS) model and stochastic declustering method, a retrospective study was conducted to investigate whether long-term seismieity anomalies existed before the May 12, 2008, Wenchuan M_S8.0 earthquake, and to evaluate the current potential risks of strong earthquakes in the Sichuan-Yunnan region. We analyzed the background and clustering of earthquakes above M_L3.0 in the period from 1977 to the day before the Wenchuan M_S8.0 earthquake, the results showed that seismic activity in the Sichuan-Yunnan region was significantly clustered, heterogeneously distributed in space and time, and the earthquake sequences were usually preceded by foreshock events. Through the analysis of the spatial variation of the 'clustering seismicity ratio', we found that there existed a long-term and large area of low 'clustering seismicity ratio' in the middle-south section of Longmenshan faults, which implied that this region had been in a state of stress barrier before the Wenchuan M_S8.0 earthquake. Statistics of several parameters including cumulative numbers of background and total sesimicity, b value and newly defined △b value were conducted to evaluate the current potential risk of strong earthquakes in the areas of low "clustering seismicity ratio' in the Sichuan-Yunnan region. The results showed that the seismic activity is correlated significantly between the Longriba and Longmenshan faults, and that the Qiaojia region and the southern segment of Longmenshan faults have high potential of strong earthquakes. Moreover, we also found that the b value trends to reflect the local variations of stress field, while △b can reveal sensitively the relative variation of the stress field in a larger spatial range.