This paper is devoted to develop an efficient method for the two phase displacement of oil and water in porous media. The pressure is approximated by mixed finite elements, while the saturation by standard finite elements with characteristics discretization.We use a much larger time step in discretized pressure equation than that in discretized saturation equation. We can reduce the numbers of saturation matrix factorization to one per pressure time step, so that a reduction of operation counts fan be obtained.It is shown that optimal order error estimates are derived under certain reseanable constraints between the discretization parameters.