A time-domain semi-analytical solution of nonlinear Schrdinger equation (NLSE) was obtained by using perturbation method (PM). And a modified perturbation method (MPM) was brought forward with the increase of nonlinearity. The numerical simulations of its applications in a high-speed nonlinear fiber-optic system,which is made up of multi-span each including one transmission fiber,one dispersion compensation fiber and an erbium doped fiber amplifier,proves that the MPM give a more precise semi-analytical solution of NLSE,which provides a novel solution to study the high-speed long-haul fiber-optic system.