Abstract: Adopting density functional method B3P86 and cc-PV5Z and setting different electric fields, the geometric structures of MgH molecule were optimized, and the bond lengths, dipole moments, vibration frequencies, IR intensities and other physical property parameters were obtained. Using the energy of high precision coupled cluster method CCSD(T) and the same basis set to scan single point energies, the potential energy curves of different external fields were gotten. The results show that the physical property parameters and potential energy values change with the external electric fields, especially at reverse direction electric fields. In order to get the critical dissociation electric parameters, the dipole approximation was adopted to construct potential energy function model, then the model was put to fit the corresponding potential energy curves of external electric fields. It is found that the fitted critical dissociation electric parameters are consistent with the numerical calculation values and theoretical analysis results, and the relative errors are less than 3%, so the constructed model is reliable and accurate. These will provide important theoretical and experimental reference for further studying the molecular spectroscopy, molecular dynamics and molecular cooling with Stark effect.