We proposed and demonstrated a new concept of multi-wavelength-switchable single-longitudinal-mode fiber laser based on the linear cavity. In the cavity, a Sagnac loop filter with a long high-birefringence fiber is incorporated to enable the allowed oscillating wavelength compatible with ITU-T grid, a multi-section high-birefringence Sagnac loop filter is used to realize wavelength-switching operation, and a saturable absorber loop filter with unpumped Erbium-doped fiber is applied to achieve single-longitudinal-mode operation. The experiment has demonstrated 16-wavelength-switchable, 100GHz spacing, single-longitudinal-mode lasing operation with a large (>40dB) output signal to background noise ratio.