To achieve single wavelength output, a single wavelength fiber laser based on a single-mode quartz-erbium-doped single-mode quartz hybrid fiber interferometer (HFI) was proposed. Based on the optical field interference principle and optical waveguide theory in hybrid dielectric fiber waveguides, the mode interference theory of HFI was analyzed, and the variation of the interference optical characteristics in HFI with the length and dislocation amount of the intermediate erbium-doped fiber (EDF) was studied. HFI was prepared using a melt dislocation method, which optimized the length of the EDF and the offset between the single-mode fiber and the EDF. The results show that, the optimal EDF length is 15 mm, and the optimal offset between the single-mode fiber and the EDF is 7.9 μm. Selecting an EDF HFI with a length of 15 mm as a mode selector for the resonator of a ring fiber laser can achieve a relatively stable single wavelength fiber laser output. This result is helpful for the application of the laser in optical fiber sensing and communication systems.