A new theory and algorithm for assembling a circular metro shield tunnel segment by using isosceles wedged rings are proposed to improve the tunnel construction efficiency and segment assembling precision. Main conclusions are drawn as follows: 1) It is theoretically proved that the tunnel axis is on a plane when rings are alternately rotated about the same angle θ and the tunnel radius R=L/(2 sin α/2 cos θ/2) where L is the ring′s width and α is wedged angle. 2) An algorithm, using allowable rotation angle method, is given to compute the position and orientation of every lining ring in the whole circular shield tunnel. 3) Every isosceles wedged ring can be identified as a left bend ring or a right bend one according to its orientation where the Ksegment is on the top of the tunnel, which means that the amounts of left and right bend rings can be respectively foreknown before constructing a shield tunnel.