Lateral coupled-mode theory of optical waveguide includes orthogonal coupled-mode theory and nonorthogonal coupled-mode theory. However, the two kinds of coupled-mode theories do not have unified analytical solutions. And they both do not deal with the asymmetric coupling power of unmatched-coupled waveguides deeply. On the one hand, a new kind of nonorthogonal coupled-mode equation is derived from the Helmholtz wave equation in this paper. And both of the orthogonal coupled-mode equations and the nonorthogonal coupled-mode equations are processed and solved uniformly. As a result, a unified solution is obtained. On the other hand, the asymmetric coupling power of unmatched-coupled waveguides is studied in detail, based on the obtained solution. The calculated results show that both of the maximum mutual-coupling power and the minimum self-coupling power can be approximated by the unified solution.