Karst detection is one of the important detection tasks in subway construction. Signal propagating characteristics of ground penetrating radar in the cave and in the rock show significant differences. The amplitude, wavelength and frequency of the radar wave change with the electrical properties of the media, and hence correct identification of radar wave group characteristics of karst cave is the key to identifying caves and their properties. This paper deals with the implication of karst detection in Nanjing Metro Line between the Binjiang Road Station and Wutang Village Road Station,with emphasis placed on the waveform display characteristics of the karst cave on the ground-penetrating radar detection. In combination with drilling verification,the authors hold that the ground penetrating radar is effective in detecting the karst in the city subway tunnel.