In the border area of Hebei, Shandong, and Henan provinces in north China, moderate-to-strong earthquakes have occurred frequently including the Cixian M_L7.5 earthquake of 1830 and the Heze M_L7 earthquake of 1937. In recent years, moderate earthquakes have occurred frequently in this region, especially at the border area of Puyang, Henan Province and Juancheng, Shandong Province. Clusters of small-to-moderate earthquakes in this region have been referred to as "small earthquake concentrating area in Puyang". Four earthquakes of M_L≥3 have occurred in this area since the 2011 Fanxian earthquake(Oct. 13, M_L4.3) with a frequency of 1, thus indicating a significant in decrease in seismicity in this area. Earthquakes of M_L≥2 have occurred more frequently since Nov. 2013, including the two Fanxian earthquakes(Nov. 20, 2013M_L3 and Aug. 5, 2014M_L3), thus indicating increased seismicity. These frequent small and moderate earthquakes provide primary information of regional tectonic stress field. In this study, we evaluated the stress field of this area from focal mechanism solutions obtained by applying Snoke's algorithm to the data collected from the Seismic Network Centers of Shandong, Hebei, and Henan provinces. We can better understand the Liaokao fault zone by analyzing pre-earthquake characteristics, such as spatio-temporal variation of principal compressive stress axis(especially P axis) and the consistent parameters of focal mechanism solutions of moderate-strong earthquakes, as well as the background and the seismogenic structure of this region. These results are also valued as a good reference for earthquake risk estimation, rapid earthquake damage assessment, and determination of correlation between characteristic variation of tectonic stress field and earthquakes in this area. The results show that the focal mechanisms are predominantly strike-slip fault solutions, and tectonic stress features horizontal or near horizontal torsion. Further, the azimuth of the P axis is related to seismic activity, and the temporal variations of P axis can be used as a reference for predicting small and moderate earthquakes in this region. Pre-earthquake spatio-temporal variation is a complex process that varies between different regions; therefore, further research is required on more cases to obtain more effective solutions for earthquake disaster prevention.