In order to solve the mutual restricted problem between modulation bandwidth and high Q value of microring electro-optic modulator which used intracavity index modulation mechanism in the device, an ultra-compact coupling modulation based electro-optic modulator was proposed. This device consists of trench-based frustrated total internal reflection couplers and 90° waveguide bends. A time domain dynamic model was used to evaluate the device performance, and the results show that the modulation phase change is less than 0.2 π, the state modulation depth is greater than 0.96 (normalized maximum of 1.0), and the modulation bandwidth is greater than 100 GHz. This high speed modulator exhibits a small chip area of 10 μm×15 μm and a lower power consumption. The compact device configuration shown here is beneficial to the extension at two-dimensional directions, and may be potentially utilized in large-scale photonic integrated circuits.