A tunable erbium-doped fiber (EDF) laser with a cascaded structure consisting of in-line Mach-Zehnder interferometer (MZI) and traditional MZI is proposed. The transmission spectrum of the in-line MZI is modulated by the traditional MZI, which determines the period of the cascaded structure, while the in-line MZI’s transmission spectrum is the outer envelope curve of the cascaded structure’s transmission spectrum. A stable single-wavelength lasing operation is obtained at 1 527.14 nm. The linewidth is less than 0.07 nm with a side-mode suppression ratio (SMSR) over 48 dB. Fixing the in-line MZI structure on a furnace, when the temperature changes from 30 °C to 230 °C, the central wavelength can be tuned within the range of 12.4 nm.