Abstract: In order to study the changing process of Barium strontium titanate (BaxSr1-xTiO3， BST) from cubic phase to the tetragonal phase with Ba doping and the ferroelectric characteristic of BST, the total energy of BST with different mole ratio of Ba/Sr when Ba doped was calculated and the fine structure determined， based on general gradient approximation， by means of ultrasoft pseudopotentials plane wave method. It is demonstrated that in BST, the cell volume expands and the value of c to a increases when Ba doped, which cartributed to the separation of positive and negative ions and selfpolarization. The tetragonal Ba0.8Sr0.2TiO3 behaved as ferroelectric when the offcenter displacement of Ti was up to 8 pm along \001\ direction.