To flatten the Gaussian spectrum of superluminescent diodes (SLDs), a fiber-optic spectrum equalization filter based on an extrinsic Fabry-Perot interferometric (EFPI) cavity is proposed. By the proper usage of a low-finesse EFPI cavity and the delicate design of a splitting and recombining fiber-optic path, the Gaussian spectrum of an SLD can be effectively flattened, which is verified by a numerical simulation and an experiment. An SLD with a center wavelength of 1568 nm and a 3-dB spectral width of 98 nm is spectrum-equalized to obtain a spectrum flatness of 0.27 dB in the wavelength range 1544.8-1605.6 nm. When the SLD is with a max power of 3.38 mW, the output power after the fiber-optic spectrum equalization filter can reach 178.06 μW.