In order to produce the orthogonally and linearly polarized dual-frequency laser output at 1550 nm, a two-wavelength single-longitudinal-mode erbium-doped fiber (EDF) laser with a structure of compound ring cavity is designed, which employs a polarization-maintaining fiber Bragg grating as the wavelength selector, and uses a saturable absorber of an unpumped EDF as laser single-longitudinal-mode selector. The basic principles of single longitudinal-mode selection of both compound ring cavity and unpumped EDF saturable absorber have been briefly introduced, the effect of the unpumped EDF length on the single-longitudinal-mode selection is theoretically analyzed, and the two-wavelength laser oscillating characteristics are investigated experimentally on different longitudinal-mode selection occasions. The experimental results show that the EDF laser with a compound ring cavity in which both a polarization-maintaining fiber Bragg grating and an unpumped EDF saturable absorber are included can steadily output orthogonally and linearly polarized two-wavelength single-longitudinal-mode laser at 1550 nm, and the wavelength spacing is approximately 0.344 nm. Such a two-wavelength single-longitudinal-mode fiber laser will find wide applications in the fields of laser sensing and measuring system, dense wavelength division multiplexing (DWDM) optical fiber communications, and so on.