The method B3LYP of the density functional theory （DFT） at 6-311++G（d, p） level has been used to obtain equilibrium structure of the ground state of CaS molecule, optimized parameters, dipole moment, charge distribution, HOMO energy level, LUMO energy level, energy gaps, infrared spectrum and harmonic frequency under different external electric fields ranging from -0.03 a.u. to 0.045 a.u. The results shows that with increasing the external electric field, the molecular geometry becomes strongly dependent on the field strength and behaves asymmetrically to the direction of the applied electric field. At the same time, the bond length and dipole moment μ of the ground state are changed from decreasing to increasing. At F=0.02 a.u., the minimum of bond length and dipole moment are 0.2289 nm and 1.5969 D. HOMO energy level and LUMO energy level are changed from increasing to decreasing. A decrease of the total energy gaps are found in the process of increasing the external electric field, which tells that the molecule is excited easily under a specific electric field. Excitation energies and oscillator strengths are affected by the external electric field. These results are useful for the study on the electroluminescence of CaS molecule.