A simple flat-top all-fiber comb filter based on two high birefringence fiber (HBF) Sagnac loop filters is presented. The proposed flat-top comb filter consists of two HBF Sagnac loop filters with two polarization controllers (PCs) and a fiber circulator. According to the theoretical analysis, with proper settings of the polarization state of the PCs, the comb filter can realize flat-top passband and the channel spacing also can be switched when the comb filter is convex spectrum. The 0.3dB bandwidth of the flat-top passband is 0.49nm with a free spectral range of 1.4nm. The maximum extinction ratio is nearly 20dB. The comb filter with switchable channel spacing can be obtained from 0.7nm to 1.4nm.