The invention provides a novel optimization mechanism for a gain spectrum of a Raman amplifier. By adopting the optimization mechanism, the pumping power and the wave length of the Raman amplifier can be rapidly selected to provide gain flat enough, so that the transmission capacity of an optical signal is increased. According to the optimization mechanism, a novel optimization algorithm is adopted to find an optimal solution, namely, the pumping power and the wave length for making the Raman gain spectrum most flat.