We use the group theory and molecular orbital theory to systematically study the origin of magnetic moment of cation-vacancy in un-doped magnetic semiconductors, and illustrate the mechanism of exchange-coupling between magnetic moments by Heisenberg model. It is found that the magnetic moment is related to the number of unpaired electrons, and the distribution of defects energy level is correlated closely with the symmetry of vacancy crystal field. The exchange-coupling coefficients J0 is estimated by the energy difference between antiferromagnetic and ferromagnetic states. And J0 can be used to predict the magnetic coupling. Positive J0 means the ferromagnetic coupling between magnetic moments, otherwise the coupling is antiferromagnetic. Moreover, we indicate that reduction of degeneracy of defect energy-level bears a direct relationship to the electron number occupied in the defect energy-level orbital, and therefore results in the structure distortion (John-Teller effect) of a cation-vacancy.