In recent years,the Changning Shale gas exploration in the Southern Sichuan Basin has experienced frequently small earthquakes,as well as a number of medium-sized earthquakes with M_L>4.0. Since the risk of disaster continues to increase,it has become an urgent scientific problem to analyze the evolution trend and development law of seismic risk in shale gas development zone by means of microseismic monitoring. From 13 portable near-field temporary seismic stations and national fixed network,more than 10,000 seismic events were picked up from February 2017 to June 2018. The maximum likelihood estimation method is used to analyze the seismic catalogue after double difference precise positioning and the evolution characteristics of b value in Changning shale gas development zone are put forward. The overall b value is estimated to be 0.98±0.02,slightly higher than previous results. The bilinear characteristics and differential distribution of the fitted line indicate that there exist obvious spatiotemporal differences in the evolution of b value,which has been verified with subsequent analysis. We found that the 5 periods of rapid increase of seismic events just corresponded to the oscillation characteristics of b value and the stable low value distribution of SCL value of relevant spatial distance,which was suspected to be associated with the nearby shale gas fracturing exploitation. The decrease of b value before the occurrence of four large magnitude events corresponds to the stress accumulation process before the earthquake.