Abstract: Alignment parameters were estimated and optimized to deal with the subway beyond-limit problem based on the coordinates of tunnelling centerline. Based on the coordinate of metro centerline, the capability of dealing with complicated beyond-limit situations can be enhanced greatly by estimating and optimizing the alignment parameters to satisfy the design constraints. The theory of realigning the design centerline to control the borders beyond-limit was expounded, and a realignment optimization model including objective function and constraints was put forward. Alignment fitting segments composed of line and circular curve were made on the basis of beyond-limit threshold using the orthogonal least squares. Furthermore, an optimization algorithm was put forward to combine these segments appropriately to produce an initial alignment. By optimizing the fitting segment points iteratively, an optimized alignment fitting the tunnel trajectory best and satisfying the design specifications and invasion threshold requirements was obtained. Finally, examples were given to prove the effectiveness of the algorithm. The research results indicate that the algorithm can not only optimize the metro horizontal realignment, but also effectively estimate the existing line parameters.