The effects of cerium oxide on the dielectric properties and tunable properties of barium strontium titanate/magnesia (40wt%Ba0.6Sr0.4TiO3-60wt%MgO) compound ferroelectric material for improving the properties of phase shifter were investigated. The mechanism was also presented and discussed. The XRD patterns analysis showed that with increasing quantities of cerium oxide x (wt%), the lattice constant of BST-MgO material first increased to x=0.4, then changed a little till x=1.2. Further additions of CeO2 decreased the lattice constant. The dielectric properties at 10 kHz and microwave frequency were studied. The addition of CeO2 influences the dielectric constant, loss tangent and tunability greatly. The permittivity decreased with increase of cerium oxide nearly linearly. The addition of CeO2 effectively decreased the loss in BSTM ceramic. Optimum doping amount of CeO2 reduced the high frequency loss tangents of BST-MgO compound which also ensured the moderate dielectric constant and tunability. When the doping amount of CeO2 is 1.2wt%, BST-MgO composite has the following properties: εr = 92.64, tanδ = 0.00449 (at 2.6166 GHz) and tunability = 9.67% (at 2kV/mm), it is suitable for ferroelectric phase shifter.