0.60Ba0.6Sr0.4TiO3(BST)–(0.40−x)Mg2SiO4(MS)–xAl2O3 (x=0, 0.5, 3, 5wt%) composite ceramics exhibit excellent characteristics suitable for tunable device applications. With increasing amount of Al, the dielectric peak can be quantitatively broadened and suppressed; the “phase transition temperature”Tc or (Tm) shifts to a lower temperature. Meanwhile, the tunability is still high in a wider temperature range. Far from Tc, pyroelectric effects are observed by using the Byer and Roundy technology and Slim polarization hysteresis loops are observed under high ac dielectric field at 10Hz. These proved the existence of spontaneous polarization in certain possible orientations in a broad temperature range above Tc in the paraelectric medium and reveal why 0.60BST–(0.40−x)MS–xAl2O3 have such remarkable dielectric nonlinearity.