Aiming at the problem of high insertion loss of current electro-optic modulator, a photonic crystal electro-optic modulator based on a width modulation (WM) cavity is proposed in this paper. The device is composed of a nanowire waveguide at the input port, silicon-based photonic crystal waveguide and a WM-type resonator. The connection between the first two structures uses a tapered structure to reduce the cascade loss between two different waveguides. According to the time-domain coupled mode theory and plasma dispersion effect, WM-type resonant cavity and PN doping structure are used to realize the modulation of transverse electric(TE) mode. The performance is simulated and analyzed by two-dimensional finite difference time domain method. The simulation results show that the device can realize the TE mode narrow-band on-off modulation function with a center wavelength of 1553.91 nm when the modulation voltage is 1.24 V. The insertion loss is 0.16 dB at 1550~1560 nm, and the extinction ratio is 19.73 dB. The modulation depth is 0.9894, the Q value reaches 1.5×104, and footprint is about 20 μm×9 μm.