A tunable optical filter(TOF) based on MEMS technology was designed. The broadband optical signal was first dispersed by a grating and then the wavelength corresponding to the passband of the TOF was selectively reflected to the output by a MEMS torsion mirror, thus the optical filtering and wavelength tuning functions were realized. A single mode fiber(SMF) was employed as the input and a multimode fiber (MMF) or few mode fiber(FMF) was employed as the output, thus the passband of the TOF was characterized by narrow-band and flat-top. After parameter optimization, the simulation results show that the 0.5 dB and 20 dB bandwidth are 0.95 nm/0.29 nm and 1.39 nm/0.69 nm with MMF and FMF employed, which meet the requirements by DWDM systems of 100 GHz and 50 GHz channel spacing, respectively. The output is MMF or FMF, the optical signal passing through the TOF cannot be further transmitted in SMF. The output can just be received by a photo detector, thus this TOF is usually applied as the outputs of the nodes in all optical network.