Abstract: It is difficult to determine focal depth via phase onset time fitting method for the regions where only sparse seismic stations are available. we demonstrate that focal depth can be well constrained with depth phase sPL which is sensitive to focal depth but insensitive to epicentral distance. With 2014 Shanxi reservoir earthquake swarm as a case study, this paper determined the focal depths of earthquakes by sPL phase with single station. The result show that the focal depths derived from sPL phase are well consistent with the results from CAP method and Hyposat method. The depths of the main shock and six M_L＞3.0 events are predominantly around 4−6 km, which is coincident with results of surveying and prospecting of active fault. Therefore sPL phase can be applied to small earthquakes for sparse network as it is well observed with small epicentral distances. Robust result of focal depth can be rapidly obtained by single broadband seismic station with bias of about 1−2 km.