With the rapid development of optical communication and optical interconnection, it is of great significance to study electro-optic modulators with high modulat ion rate,small device size and easy integration.Based on silicon-on-insulator (SOI),a one-d imensional photonic crystal nanobeam cavity (PCNC) side coupled electro-optic modulator is proposed. According to the time-domain coupled mode theory,bus waveguide and one-dimens ional photonic crystal nanobeam cavity form side coupling structure.The one-dimensio nal photonic crystal nanobeam cavity with cylindrical tapered aperture enables bette r confinement of light beam in the cavity.By adjusting the structural parameters of the nanobeam cavity,the working wavelength within the communication band is selecte d.At the same time,according to the free carrier dispersion effect,doping is introduced on both sides of the nanobeam cavity to form p-n junction,which enables the resonant wavelen gth of the nanobeam cavity to be adjusted when a lower bias is applied,thus realizing the "on" and "off" modulation of the working wavelength light signal.Three-dimensional finite dif ference time domain (3D-FDTD) is used to analyze the optical characteristics an d electrical performance of the modulator.The results show that the insertion loss and extin ction ratio of the electro-optic modulator with a compact size of only 25μm² and a low mod ulation voltage is only 1.175V at the operating wavelength of 1550.55nm are 0.04dB,18.2dB,respectively. The modulation speed is 8.3GHz and modulation bandwidth can reach 90GHz,which can be appliedto integrated photonic devices andhigh-speed optical communications fie ld.