The complete Hamilto matrices of trigonal symmetry 3d4/3d6 electron configuration are derived using Racah′s irreducible tensor operator method in the weak field scheme. The ground-state energy levels and crystal structure of the Fe2+ in Fe2P2S6 crystal are studied and the ground-state energy levels and the zero-field splitting of electron paramagnetic resonance of Fe2P2S6 crystal are calculated considering the effects of spin-orbit coupling and spin-spin coupling in ground-state energy levels. The calculated results are in good agreement with the experimental values. Moreover, the effects of spit-orbit coupling, spin-spin coupling and low-spin states 3L on the ground-state energy levels and zero-field splitting are investigated further based on the calculated results and the Jahn-Teller effect of Fe2P2S6 crystal is studied as well.