Polycrystalline Cd0.7Fe0.3Cr2S4 has been prepared by a solid state reaction. A crystalline structure study shows that the x-ray diffraction pattern can be indexed successfully in a cubic spinel phase. Its magnetic and dielectric properties have been investigated. A ferrimagnetic phase transition of the material was identified at 123 K. Larger permittivity and loss tangent are observed than those of CdCr2S4 in the frequency range 300 Hz–1 MHz. An anomaly cusp induced by the internal magnetic field on ε-T and tan δ-T curves is observed near its magnetic transition point, implying an existence of magnetodielectric effect. At low temperature T≪TN, the permittivity variation Δε depends on magnetization M and shows clearly a linear relationship between Δε and M2, indicating an interplay of permittivity and magnetism. Furthermore, compared to CdCr2S4, Cd0.7Fe0.3Cr2S4 exhibits smaller values of dielectric relaxation time and activated energy above TC due to Fe2+ doping.