The depth distribution of aftershocks of the 1996 Lijiang M_S7.0 earthquake is strongly time-dependent. Events occurring shortly after the main shock had deeper focal depths, and as the time going on, the focal depth of the aftershocks became shallower and shallower, i.e. the cutoff depth of seismicity became shallower and shallower with time. As we know, the lowermost events occur around the depth of brittle-plastic transition, and this depth depends on strain rate. The postseismic deformation model inferred from GPS data show that the major contribution of postseismic strain release comes from the lower layer of the crust. These results suggest that significant afterslip is related to viscous relaxation of lower layer. We estimated the lower bound of the strain rate according to Marone's et al.(1991)afterslip model and the slip data observed at the surface of Xianshuihe Fault. The results show that the strain rate is high after the main shock, and decreases gradually with time. We calculated the strength profile of middle crust based on flow law of wet quartz, estimated strain rate, temperature profile determined using the heat flow data at Lijiang, as well as crustal structure based on P wave velocity. By comparing the cutoff depth of seismicity and the brittle-plastic transition depth of the middle crust, we found the two depths are consistent to each other. We suggest the temporary existence of deeper small events after main shock and the depth distribution of aftershock is due to the changing brittle-plastic transition of the middle crust corresponding to strain rate variation from high to lower values after the main shock, and this kind of change is the manifestation of rheology of the middle crust.