The boundary region between Alxa Block and Ordos Block is an area of stress concentration with strong seismicity and frequent small earthquakes. However, the knowledge of this area is limited since only a few seismic stations were deployed in this area. The 2015 Ms5.8 Alxa Left Banner Earthquake on April 15 is the largest one occurred in the surroundings since the 1976 Ms6.2 Bayinmuren Earthquake. Abundant stations built in the northern part of Chinese North-South Seismic Belt recorded this event sequence well within short distances, which provides us a great opportunity to carry out studies. We use these data to obtain a mean 1-D layered velocity structure via iterative inversion based on both travel time and waveform misfits. Then we use the travel time difference between data and synthetic seismograms to relocate the epicenter. Finally we invert the best double-couple focal mechanism and centroid depths of the source. As the result, the source was located at (39.7663°N, 106.4304°E) with a depth of 18 km and Mw 5.25. Nodal plane 1 had strike 176°, dip angle 85°and slip angle-180°, while plane 2 had strike 86°, dip angle 90°and slip angle-5°. Considering the tectonic characteristics of regional fault zone, we believe this earthquake was caused by a nearly pure left-lateral strike-slip fault with a slight normal component, while the nodal plane 2 striking towards NEE (near E-W) was the fault plane. The seismogenic structure was likely to be an E-W striking buried fault nearby. From our study, the corresponding fault of this event may indicate all groups of faults with same E-W strike has the common character of large-dip left-lateral strike slip. Moreover, there may be some buried faults being newly born or not found yet. These results could be an important supplement to the future research of regional seismicity and modern fault zone structure.