This paper introduces the law that the capacitivity of NaCl solution changes with different salinity, different concentration and different measurement frequency. The experiments show that capacitivity of NaCl solution increases in square law with the concentration increasing, decreases in negative exponent law with the frequency increasing and, in negative exponent law with the pole distance increasing. Comparing with resistance of NaCl solution, it shows capacitivity change larger than resistance under the same condition. Based on the results of these experiments, this paper provides a new concept, i.e. the secondary dielectric constant whose distinctive difference with dielectric constant is that it directly detects the dielectric characteristics of electrolyte. This paper also provides a new exploration method, surface capacitivity exploration. The new instrument can be improved on the dual-laterolog apparatus. By this method, it can not only detect oil or gas reservoir, but also detect other mental ores.