In this study, the transmission characteristics of a double Sagnac loop filter, composed of two segments of PMF in parallel, are analyzed, simulated and measured in detail, which proves that the proposed double Sagnac loop filter has the polarization-independent characteristic and the tunable channel spacing. According to the results of simulation and measurement, a channel spacing tunable multi-wavelength erbium-doped fiber laser utilizing a double Sagnac loop filter with the assistance of the FWM effect is designed. The laser can output multi-wavelength lasing lines with channel spacings of 0.35 nm and 0.9 nm. Owing to the polarization-independent characteristic of the double Sagnac loop filter, the polarization-dependent devices are not interfere with the transmission spectra of the filter. Thus, the double Sagnac loop filter exhibits the simple structure, polarization-independence, the tunability of channel spacing, and a great potential application in the field of multi-wavelength fiber lasers.