Abstract: An new algorithm-orthogonal trinocular stereo with SMW （symmetry multi window）, is presented in this paper, which is based on the disparity estimation uncertainty of the efficient method in binocular stereo SMW. Using the disparity estimation uncertainty, the disparity maps of horizontal stereo pair and vertical stereo pair from SMW method are fused into a new one. In the implementation of this method, a fast algorithm of binocular SMW is put forward, which makes the time of computing nine windows and two directions （from left to right and from right to left） ZNCC equivalent to that of computing one ZNCC . The time is reduced to 1/18 of the original. Experiments with read trinocular stereo images have been done and the results show that comparing with the binocular SMW and other similiar methods, the new algorithm yields robust disparity estimation in the presence of occlusion and smooth region in single direction, and the speed of computing is satisfactory.