Based on earthquake observation report recorded by the Zigong local digital seismic network and mobile stations from January 2007 to August 2010, the earthquake relocation and velocity structure inversion are developed in the Zigong and Longchang area. Taking into account the time series of seismic activities and injection pressure of water injection well （Jia 33 well） in study area, we discuss the focal depth distribution, the characteristics of P-wave velocity structure and relationship with injection pressure of Jia 33 well in and around injection region. The positioning result shows that the earthquake space cluster distributions are more obvious, the earthquakes are concentrated in three cluster regions of Zigong （region A） , Fushun-Longchang juncture （region B） and Longchang （region C）. Region A is about 20kin to the west of Jia 33 well, earthquakes distribute along NNE direction, and the dominant range of focal depth is 5 15kin. Earthquakes distribute along NW direction in regions B and C. Region B is where Jia 33 well locates in, the focal depth of most earthquakes are between 2km and 6km, which is near the affusion layer of Jia 33 well. The focal depth presents shovel-shaped distribution characteristic which spreads from NW to SE, and then become gradually deep. Region C is about 15kin to the southeast of Jia 33 well. The focal depth in region C is relatively deeper than that in region B. The dominant depth is 8 - 15kin. The seismic activities of region A are independent of injection pressure of Jia 33 well. However the injection pressure of Jia 33 well has obvious regulatory effect on seismic activities of Rgions B and C. The P-wave velocity structure shows that near the affusion layer at the depth of 3km, there is a near NS direction transition zone of high-and-low velocity between regions B and A. P-wave velocity in region B is higher than that in region A. We make the conclusion that the significantly increased P-wave velocity in region B results from the higher water saturation of underground medium by square-cavity saturation of Jia 33 well and it has a correlation with injection pressure of Jia 33 well.