Manganese doped cadmium ferrites (Cd1-xMnxFe2O4, x=0, 0.25, 0.5, 0.75, 1) were studied by using ab initio calculation based on generalized gradient approximation (GGA). The dependence of electronic structure and magnetic properties on the additional on-site Coulomb repulsion (U) was investigated. In the calculation, normal spinel structure is assumed. Exchange interactions between magnetic ions are calculated from the total energies of ferromagnetic and other spin configurations. The lattice constants given by GGA show a linear dependence on the concentration of Mn (x) . The local magnetic moments of Fe and Mn ions decrease as x increases. The exchange integral JAB is negative and its magnitude increase as x increases, while JBB1 shows a transition from antiferromagnetic to ferromagnetic value.