On the basis of our previous experimental results for the diamond synthesized in FeNi-C system by B and N co-doping under high pressure and high temperature conditions, the crystals doped with B or N atom, and co-doped with B and N atom are investigated separately by using the first principle density functional theory based on the stable structure of diamond. The formation energies corresponding to the all doping diamond crystals are studied while the most stable co-doping structures with the lowest energy are obtained. Furthermore, the absorption spectra, the densities of states, and the electronic structures of the doping diamond crystals are discussed. The absorption spectra show obviously the red shift, which is consistent with the experimental result. The electronic results show that the doping of N atom is prior to the doping of B atom while the system co-doped with nearly neighboring B and N atoms creates the most stable structure with the lowest formation energy due to the synergistic effect.