Diamond nucleation is the key of synthesizing diamond films. At present, the negative bias is the most effective method of enhancing diamond nucleation. In this work, the forming energy is emphatically calculated that a diamond nucleus is built up on a micro defect (pit) produced on the substrate surface by ion bombardment in the process of negative bias enhanced diamond nucleation. The analytic function between nucleation energy of diamond, nucleation density, nucleation speed and pit density is given in some reasonable formulas. The results indicate that descent of the nucleation energy of diamond with the pit density resulted in enhancement of nucleation density and nucleation speed, and the results are in agreement with experimental results in conferences. The reason of the pit reduced the nucleation energy is analyzed.