Abstract: In this paper, we have analysed the periodicity of underground water grouped anomalies before andafter the Datong earthquake by maximum entropy spectrum and Fourier method. The results showedthat the underground water grouped anomalies have a series of periodicities, and the four month periodicity is obvious, the amplitude of the subwave is distorted before strong earthquakes. By using themethod we may judge the anomaly nanture (as earthquake, no earthquake), and judge the complex relation of the anomaly to the eathquake, proved that the method has important practical sense to judgeearthquake situation and predict earthquake.