Abstract: In this paper, an example of velocity ratio anomaly of seismic velocity is introduced.Using the method of velocity ratio and criteria for the identification of anomaly defined in the paper "GUIDE TO THE PROCEDURES OF METHODS FOR EARTHQUAKE PREDICTLON IN SEISMOLOGY", the data of velocity ratios of small earthguakes recorded at the Shanxi seismic network before the Ms 6.1 Datong -Yanggao earthquake swarm in Shanxi Province are analysed.As a result, it is foundthe anomalous area of low velocity ratio values is formed near its epicenter in two years before the main shock, and the area is like ellipse in shape, of which the length of the major axis is about 200km, with a north-easterly trend, and the length of the minor axis about 135km.The anomalous duration of the wave velocity ratios is 17months and anomalous amplitude is -0.1657.