An in-fiber Mach-Zehnder interferometer was proposed and fabricated, which was based on a sandwich-like etched single mode fiber driven by only one acoustic transducer. It succeeded the feature of fast tuning and would not introduce frequency shift in the transmission spectrum. Based on it, a fast tuning dual-wavelength laser with a two-wavelength spacing around 3.5 nm was proved with a tuning range of about 3.6 nm, covering wavelengths from 1561.6 nm to 1568.9 nm.