For the dual-channel encryption system, based on polarization-division-multiplexing, we put forward a new control scheme for complete chaos synchronization by means of linear electro-optic (EO) effect. In the scheme, the chaotic synchronization quality of each linear polarization (LP) mode component varies periodically with the applied electric field. The variation regulation is as follows: Complete chaos synchronization ↔ acute oscillation. With the applied electric field fixed at a certain value, the robustness of the complete chaotic synchronization quality due to the bias current and the feedback strength is improved greatly by EO modulation. Each LP mode can obtain the complete chaos synchronization in a large range of the bias current and the feedback strength. And the encoding message modulated to each LP mode can be almost re-established.