La_2O_3 doped Ba_(0.55)Sr_(0.45)TiO_3/Mg_2TiO_4 microwave composite ceramics were prepared by traditional solid-state reaction,and the influence of La_2O_3 doping on microstructure,dielectric properties and tuning was investigated.The results show that La~(3+) enters BST lattices when the mass fraction of La_2O_3 reach 1.2%,and La~(3+) doped sample can prevent the change of the chemical valence of Ti in BST/Mg_2TiO_4 composite system from +4 to +3.Mg_2TiO_4 and La_2O_3 doping can reduce the dielectric const...