As the video vehicle detecting　has a great importance to the intelligent transportation system, a new moving vehicle detection method based on optical flow in complicated background was presented. To deal with the hypothesis that the smoothness of optical flow and the conservation of gray intensity don’t come into existence on the condition of shadow, boundary and overlap, a new forward-backward optical flow equation is introduced and the Hessian matrix of equation is computed. Then the conditional number of the matrix is combined with the associated matrix of Lucas-Kanade’s optical flow method to eliminate the restriction points which are not credible within local neighborhood. Experimental results show that the proposed method helps to obtain satisfying detecting results due to its good stability and high detecting accuracy. The results can be used as the basis of advanced vehicle control and traffic management in intelligent transportation systems.