In this paper a parameter called “concentration ratio of distribution of P axis” was defined to analyze the variation of focal mechanisms of small earthquakes before a moderate-strong earthquake.This parameter shows the dynamic change of the direction of the P axis in the seismogenic process. In this paper，we use two moderate strong earthquake examples，namely the Jingtai earthquake of on Jun. 6，2000 and the Sunan earthquake of on March 30，2008. We invert the focal mechanism solutions and the seismogenic stress field using the joint inversion method of first motion and amplitude ratio of P and S wave for small events before the two moderate strong earthquake examples.The data used are from the networks set by China and France and the Gansu digital monitoring network.We study time-varying characteristics （prior to the two earthquake examples）using the parameter “concentration ratio of distribution of P axis”. The results show that the P axis of the focal mechanisms for events begin to concentrate one or two years before the main earthquakes and in accordance with the main earthquakes’ source mechanism just prior to the main earthquake.The concentration ratio of distribution of P axis prior to the Jingtai earthquake varies regularly；moreover，the parameter prior to the Sunan earthquake began to show concentration from the beginning of 2006，whereas the concentration ratio decreases from the beginning of 2007 and increases a greater degree by the end of 2007 until the main shock occurred. This could potentially be due to the singularity of the seismogenic fault structure of the Jingtai earthquake or the complexity in the epicenter region of the Sunan earthquake.For the Jingtai earthquake，the location stress field would be regionally adjusted during the concentration process and followed by strengthening of the local stress field，which makes the source mechanisms of medium and small earthquakes uniform.