Abstract: 〖WTBZ Optical power splitters are essential devices for optical fiber communication system in passive optical networks(PONs) and photonic integrated circuits (PICs). As one of the optical splitters, 12 electro-optical multimode interference (EO-MMI) power splitter fabricated by electro-optic organic polymer was analyzed by the selfimaging effect. The operation principle of the 12 EO-MMI power splitter was discussed. Firstly, the effective index of the three dimensional (3-D) ridge waveguide was computed by the effective index method (EIM). Then the positions of the two images were calculated respectively by using the formulas of the selfimage effect. The power-splitting ratio in the output waveguides was simulated by the guided mode propagation analysis (MPA). Research results show that, for the 12 EO-MMI power splitter operating at 1.55 microns, the special powersplitter ratio can be obtained if there are no electrodes on the ridge waveguide. Furthermore, when the electrodes are put on the top of the waveguide, the power splitting ratio is realized for a tunable scope of 7.8dB, for the electro-optical effect to change the refractive index.