The precisely controlled spectral characteristics of a novel Sagnac loop filter was studied, which is structured by insertting two segments of high birefringence optical fiber and a fiber polarization controller into a conventional optical fiber Sagnac loop filter. The filtering characteristics of the filter was investigated theoretically by using the equivalent optical-path analysis and transfer matrix method. Results show that when the birefringence parameters of fiber are fixed, the spectral characteristics of the Sagnac loop filter can be precisely controlled by changing the polarization controller status. The Sagnac loop filter can be used to flat the spectra of erbium-doped fiber (EDF) amplified spontaneous emission (ASE) light source, and the broadband ASE laser with flat spectra is obtained in experiment by controlling the tilt angle of the three waveplates of polarization controller.