Two up-tapers are produced on a single mode fiber with the distance of 4.5 cm by the technology of fiber waist-enlarged fusion, so as to form a new single mode structure fiber with two spherical shapes based on Mach-Zehnder interferometer (MZI). A tunable self-seeded multiwavelength Brillouin erbium-doped fiber laser is designed based on MZI and put in the glycerol solution of different mass fractions. At the wavelength of 980 nm and the pumping power of 200 mW, the multiwavelength of the output laser is tuned from 1558.6 nm to 1562.8 nm, correspondingly to the changes of the effective refractive index of the optical fiber cladding, and the tunable range is about 4.2 nm. The six Brillouin laser wavelengths with the wavelength interval of 0.088 nm and the mean optical signal to noise ratio of more than 15 dB is obtained. The proposed laser has the advantages of simple structure, stable wavelength interval, and has great application prospect in dense wavelength division multiplexing and optical fiber sensing.