The influence of La2O3-doping on the dielec- tric properties of Ba0.6Sr0.4TiO3-6wt%MgO was studied. With the increase of La2O3-doping content, the space between adjacent parallel crystal planes of BST-MgO composite first became larger, then smaller. Proper quantity of La2O3 could ensure moderate dielectric constant and reduce the dielectric loss of BST-MgO composite at high frequency, but excess quantity of La2O3 would lower dielectric constant. When the La2O3-doping content was 0.2wt%, the dielectric constant and loss of BST-MgO composite were equal to 93.2 and 0.005 (1MHz), respectively. And the dielectric constant tunability could be obtained to 12.1% (21kV/cm).