Abstract: Controlled instability of parallel shear flows induced by weak disturbances is of great academic and pragmatic importance in fluid dynamics and industrial applications. However, the underline mechanism is still not fully understood yet. Consequently, this paper intends to explore the mechanism of sound-induced shear flow instability by a lattice Boltzmann simulation. It is found that the spatial inhomogeniety of sound-wave amplitudes plays a key role in the instability process. There are two types of developing of Kelvin-Helmholtz instability depending on the wave length of sound wave, namely the long- and short-wave modes. In the short-wave mode, the Doppler effect causes deviation of wave number of the sound in the upper and lower layers, respectively. The interference between the sound waves generates sound wave packets in the shear layer, which further induces flow instability if the length scale of the wave packet matches the unstable wave length of the shear layer. In the long-wave mode, unstable waves develop below the sound source. Sound waves are reflected by the shear layer nonuniformly. The symmetric distribution of the sound pressure is broken. On the other hand, the wave length of the distributed pressure wave decreases with time due to nonlinearity.