An observation scheme, which is designed several frequencies suited over the natural audio electric field frequency range, is used to qualitatively understand the electrical structure of underground geological body. Weak signal detection technology has been used to amplify and filter the weak natural electric field signal, and read out them after selecting corresponding frequencies and integrating. This instrument can observe eight natural electric field signal at frequency of 8Hz, 16Hz, 32Hz, 64Hz, 128Hz, 256Hz, 512Hz, 1024Hz. Several reliable technologies have been used in the hardware design and software development to improve both the anti-interferences capability and system stability. Compared with field work instance, we conclude that natural audio electric field frequency selecting instrument can get the underground geological information within a certain depth, and therefore it can be used for fast survey in geology and mineral resources, and in engineering geophysical and water resources.