The precursory wave is a kind of transient signals and low-frequency waves in the process of source nucleation． It is also an important prediction value and reliable short-impending precursory information． This paper extracted 5 cases of digital low-frequency precursory wave abnormal images before 3 moderately strong earthquakes in the Heilongjiang and its adjacent area， and analyzed abnormal morphology，abnormal characteristics and abnormal index of low-frequency precursory waves． We got a new understanding on the basis of preliminary statistic analysis and precursory wave abnormal mechanization research． These earthquake cases have greatly enriched the digital low-frequency precursory wave before moderately strong earthquakes in near field region，and contribute to the application of low-frequency precursory wave on analysis and prediction work in the Heilongjiang region．