Based on the model of discrete-time linear systems, the paper builds up the state space model of cantilever assembly method by studying the stream of variation theory. To address the problem of evaluation in alignment error, the evolution index of comprehensive stability is presented after comparing the advantage and disadvantage of evaluation index of point stability and evaluation index of slope stability. The method of evolution index of comprehensive stability is demonstrated by evaluating the alignment error of a long-span cable-stayed bridge, and it can accurately evaluate the error of assembly process.