In this paper, the temporal variations of S-wave splitting before some moderately strong earthquakes in North China were studied by using two methods. The first method is a usual analysis of direct S-waves on the seismograms recorded on surface and in boreholes at Jixian, Jinghal. Wuqing stations of Tianjin Network. For increasing the distinguish ability, the statistic method of data analysis was applied too. The second method is an analysis of S-wave polarization by using the digital records of Baijiatuan Station, Beijing. The temporal variation of S-wave splitting before 1989 M=6.1 Datong earthquake was studied. The results show that the premonitory variations in S-wave splitting before some moderately strong earthquakes in North China may be observed and discovered, and it can be considered as an index reflecting the seismic velocity anomalies.