A new method by which several light beams can be synchronously modulated based on attenuated total reflection (ATR) polymer electrooptic modulator with single pair of electrodes is proposed. Absorption peaks of ATR are corresponding to waveguide resonance modes, respectively. In the experimental system, the absorption peaks of ATR are used as the modulation channels. Several light beams can be modulated simultaneously through that every incident angle is related to a work angle of different waveguide resonant modes, respectively. Double-channel modulation results using three modulation types have been demonstrated. Results show that the lower guide mode acting as the modulation channel is, the larger modulation index can be obtained. 42.9% modulation efficiency can be obtained when the lowest guide mode acts as the modulation channel at the 832 nm wavelength.