We present an all-fiber frequency shifter (AFFS) consisting of a fiber Bragg grating (FBG) modulated via an acoustic flexural wave for optical heterodyne measurement. The AFFS can efficiently generate the frequency-shifted signal due to the resonance peak with a high-reflection efficiency and being completely separated from the reflection spectrum of the original FBG, simultaneously. The experimental result shows that the minimal measurable vibration amplitude and the resolution of the all-fiber optical heterodyne measurement setup constructed with the AFFS are 0.06 nm and 30 pm in the region of tens to hundreds of kilohertz, respectively.