Abstract: Fiber filter has important applications in fiber sensor and optical fiber communication system. Especially, filter with all-fiber structure and tunable wavelength spacing is worth studying due to its unique characteristics. A polarization-independent all-fiber birefringent filter with adjustable wavelength spacing is proposed. The apparatus consists of a polarization beam splitter (PBS), two sections of polarization-maintaining fiber (PMF), and four halfwave plates (HWPs). The theoretical analysis shows that it operates as a polarization-independent multi-wavelength filter with good channel isolation and adjustable wavelength spacing by controlling the four HWPs in the loop. This filter is applicable for flexible multi-wavelength fiber laser and other optical fiber systems.