A switchable and spacing-tunable dual-wavelength linear cavity erbium-doped fiber (EDF) laser based on a polarization-maintaining fiber Bragg grating (PM-FBG) is demonstrated experimentally. Feedback from the FBG results in the laser operating on two longitudinal modes that are separated in both wavelength and polarization. The birefringence induced by the FBG is beneficial to diversify the polarization states of different wavelength in the EDF and enhance the polarization hole burning (PHB) effect. This PHB greatly increased the inhomogeneous gain broadening of EDF and accordingly reduced the wavelength competition. Then, it was possible to achieve stable dual-wavelength oscillations at room temperature. On the other hand, through adjusting the state of the polarization controller (PC), the transmission characteristic of the FBG is changed. When the eigenaxe of one mode is more populated than the other, it will oscillate stably in that mode. Especially, when the eigenaxes of both modes are equally populated, the two modes can co-exist with equal amplitudes. Thus mode selection is achieved in laser oscillation and the laser can be designed to operate in wavelength switching modes, only by simple adjustment of the PC. Applying strain in line with the fast axis reduces the birefringence of the fiber grating and causes the wavelength separation of the two modes to be decreased. On the other hand, strain applied in line with the slow axis increases the birefringence and a concomitant increase in wavelength spacing is produced. Then, transverse strain loading on the FBG allows the wavelength spacing to be controlled and a tunable wavelength separation from 0.2 to 1.1 nm was demonstrated.