The cascade process of frequency doubling and electro-optic (EO) coupling in a single periodically poled niobate (PPLN) crystal is studied. The original coupling process is disturbed by the introduction of another optical wave (control light). Numerical results indicate that magnitude of three waves and the polarization of the second harmonic (SH) can be both modulated by adjusting the intensity of control light. This result would be useful when simultaneous frequency doubling and signal tuning are desired.