The shell model interionic and interatomic potentials are widely used to perform atomistic simulations of ionic crystals. The parameters of the potentials are obtained by fitting the data from measurements and/or electronic structure calculations of the crystal discussed. A new method of fitting shell model interatomic potential is presented for diatomic molecule from electronic structure data obtained by quantum chemical ab initio calculation in the paper. And the shell model interatomic potential for H is fitted from the electronic structure data of H2 molecule for example. To examine the method, the interatomic potential and force of H+2 molecular ion are calculated from the potential functions. The results demonstrate that the proposed new fitting procedure shows a fairly good feasibility to the shell model potential for covalent bond molecule.