Swept light source is a key component of fiber optic sensing demodulation. High-quality of the swept light source can improve the signal- to- noise ratio and demodulation precision of fiber sensing system. A Fourier domain mode locking (FDML) operating mode of the swept light source is reported. Semiconductor optical amplifier (SOA), fiber-optic tunable FP filter, isolator, fiber coupler and optical delay line form the swept source ring cavity. While a filter based on Febry- Perot (FP) cavity is designed to flat the output spectrum. The swept range is 1506~1566 nm, swept speed is 250 Hz, flatness is better than 0.3 dB, full width at half maximum (FWHM) is less than 0.16 nm, the average optical output power is 4.53 mW. The swept source has the advantages of simple structure, low cost and good flatness. As the light source of optical fiber sensor demodulation system, swept source has some practical value.