This paper has discussed the method of normalized full gradient of gravity anomaly in wave-number domain and suggested the introduction of smoothing filter operator in the process of downward continuation and derivation for potential field conversion. This technique helps increase the reliability and stability of calculation and extend the depth of continuation. Based on lots of model experiments and data analysis, the authors have determined the experiential value of exponential parameter about smoothing filter operator in calculating downward continuation and derivation, and pointed out the variational regularity between the depth of the geologic body and the harmonic number N: the lower (deeper) the depth of geologic body, the bigger (lesser) the value of N. It is also discovered that there exist similar characteristics of contour G^H field between the three dimension model of symmetrical density anticline and asymmetric oil storage anticline and the two dimension model, and this offers the new thought for calculating the center of gravity and investigating the oil storage structure.