The filter mechanism of a tunable filter with narrow band in bulk semiconductor laser amplifier (SLA) is analyzed, which is suitable for dense wavelength division multiplexed (DWDM) optical fiber networks. The wavelength selectivity of the filter is derived from nondegenerate four wave mixing (NDFWM) in semiconductor gain medium. It uses the direction of propagation to separate the desired wave mixing product from the other light beams. The filters are shown to have very narrow passbands (subangstrom bandwidth) tunable over the entire semiconductor gain bandwidth. We gave the theoretical analysis of NDFWM effect in semiconductor gain medium is given and a set of nonlinear coupled wave equations is deduced, which are processed efficiently by Runge Kutta method in numerical simulation. The influences of various structural parameters of the filter on the output bandwidth and efficiency were also discussed. The filter mechanism of a tunable filter with narrow band in bulk semiconductor laser amplifier (SLA) is analyzed, which is suitable for dense wavelength division multiplexed (DWDM) optical fiber networks. The wavelength selectivity of the filter is derived from nondegenerate four wave mixing ( It uses the direction of propagation to separate the desired wave mixing product from the other light beams. The filters are shown have very narrow passbands (subangstrom bandwidth) tunable over the entire semiconductor gain bandwidth. We gave the theoretical analysis of NDFWM effect in semiconductor gain medium is given and a set of nonlinear coupled wave equations is deduced, which are efficiently efficiently by Runge Kutta method in numerical simulation. The influences of various structural parameters of the filter on the output bandwidth and efficiency were also discussed.