The acoustic-induced refraction index change in optic fiber is introduced simply. Based on the composite waveguide theory, the difference between the general fiber coupler and the null coupler is discussed, and the latter in essence acts like a section of two-mode fiber which can be exciting separately. From the coupling mode theory, the asymptotic acoustic-induced coupling-mode equations in null coupler is resolved, and a complete analytic expression for power conversion between the fibers is given out firstly. With that, the frequency shift characteristics and functions provided with this kind of device at resonance frequency are explained. In the end, an experimental study on an acousto-optic tunable splitter or a switch is reported.