The geometrical optics theory is adopted to analyze the change of light transmission mode in tapered fiber, and multimode transmission is achieved when tapered fiber diameter is in the range of 1.135-60 μm. Tapered fibers are fabricated with fused biconical taper system. The relationship between the tapered fiber radius and the stretching length is obtained by Origin software. Under the given conditions, tapered fiber with waist diameter of 2.475 μm is fabricated when the stretching time is 95 s. The transmission performances of tapered fiber with waist diameter of 2.475 μm are simulated by the BeamPROP module of RSoft. We use amplified spontaneous emission (ASE) optical source to measure the output optical power change with wavelength in 1530-1580 nm. The tapered fiber is inserted into the ring cavity, and the polarization state of the signal light in the cavity is changed by the polarization controller to realize wavelength tunable in the range of 1563.828-1565.444 nm. The side-mode suppression ratio is 51.4 dB, and the double wavelength output with a maximum wavelength interval of 1.48 nm is achieved while the side-mode suppression ratio is 49.8 dB.