Wave-coupling equations including acousto-optical (AO) polarization rotation and frequency doubling in periodically poled lithium niobate (PPLN) are proposed. The acoustic wave could either be induced from an external transducer or self-generated in PPLN driven with a cross-field radio frequency field. The proper reciprocal of PPLN is used to compensate the second harmonic generation (SHG) wave-vector mismatching, while phonon with suitable frequency may affect the pump polarization by scattering photons. Numerical simulation shows that there is a competition between the AO polarization rotation and SHG in PPLN under the phase-matching condition. The influences of the acoustic wave intensity on the polarization or intensity of three waves are studied.