The nonlinear Schr?dinger equation (NLS) including gain effect is derived from quasi-NLS equation which describes the behavior of soliton pulses in the fiber with slowly decreasing dispersion. The solution of this equation and pulse-width evolution expression are given. For step-index profile single-mode fiber, ordinary relationship between fiber structure parameters and gain coefficient are obtained. Numerical simulation shows that the fiber with slowly decreasing dispersion not only can compensate for soliton broadening caused by fiber loss, but also can compress soliton pulses.