The invention discloses a high-performance straight-waveguide-type electro-optic phase modulator and a preparation method thereof. The electro-optic phase modulator has a plate-capacitor-type structure, and comprises a single crystal which is equivalent to a dielectric layer and electrode layers on the upper and lower surfaces of the single crystal. The single crystal is a binary (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 system or a ternary (1-x-y)Pb(In1/2Nb1/2)O3-xPb(Mg1/3Nb2/3)O3-yPbTiO3 system. The straight-waveguide-type electro-optic phase modulator is capable of using the domain steering and the difference of birefringence of a ferroelectric, inducing the domain steering through a prefabricated electric field applied to coplanar electrodes, producing the change of the refractive index which is up to 3% in the local section, and effectively constructing a simple optical waveguide structure which is easily processed and manufactured. The high-performance straight-waveguide-type electro-optic phase modulator has the characteristics of very low half-wave voltage, low insertion loss, high bandwidth and excellent phase modulation ability. The ferroelectricity and the strong optical nonlinearity are combined, various optical information conversion devices can be designed, and the effective control of the photon polarization state, the detection of a weak signal and the generation and control of a phased array signal are realized. The high-performance straight-waveguide-type electro-optic phase modulator is suitable for the quantum secret communication technology and microwave photonics radar technology fields.