We report an orthogonally polarized dual-frequency continuous-wave (CW) Er∶?YAG laser that can simultaneously output laser with wavelengths of 1617 nm and 1645 nm. Polarization beam splitter (PBS) prism and etalons are inserted into the cavity to generate an orthogonally polarized dual-frequency mode. The maximum output power of the S-polarized 1645 nm and the P-polarized 1617 nm lasers is 1.074 W and 0.242 W, respectively. We measure the longitudinal mode spectrum of the output laser using a Fabry?Perot (FP) interferometer and prove that the lasers at both wavelengths are single longitudinal mode. The M² factors of the 1617 nm and 1645 nm lasers are 1.32 and 1.87 in the x direction and 1.38 and 2.06 in the y direction, respectively, and the frequency difference is 3.28 THz.