In the preparation process of a strong earthquake,at the potential source area and its vicinity,the tectonic stress field can be enhanced by local tectonic activities.There will be a space-time adjustment of the stress field with the background of a stable regional stress field.To detect the crustal stress state of the study area,in this paper we analyze the focal mechanism data of small to moderate earthquakes that occurred at the epicenter of the Lushan M_S7.0 earthquake and its surrounding areas more than ten years before the Lushan earthquake.We obtained the focal mechanism solutions of the small to moderate earthquakes using the Hardebeck and Shearer (HASH) method,then used the C_V value of the P-axis azimuth (the ratio of the standard deviation to its average) to identify the stress field change.The results show that the stress field obviously changed at the seismic source and its surrounding areas with time.The P-axis azimuth C_V values of the small to moderate earthquakes exhibited a rise-decline-rise process prior to the Lushan M7.0 earthquake,which means that there was a long period of stress accumulation before the M7.0 earthquake,consistent with the results of many previous studies.The C_V values and apparent stress had the same tendency changes,which objectively reflect the changes in the stress field in the process of earthquake preparation.This study demonstrates that there is a certain relationship between the occurrence of strong earthquakes and the change in the P-axis azimuth C_V values of small and moderate earthquakes.