Electro-optic modulators, which convert electrical signals onto the transmission light, are key devices in electro-optic modulating systems. Modulation efficiency is one of the most important parameters of an electro-optic modulator, which directly determines the footprint and power consumption of the device. Generally, modulation efficiency strongly depends on the electro-optic response of the crystal. The Pb(In_1/2Nb_1/2)O₃–Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PIN–PMN–PT) single crystal with giant electro-optic coefficient ( ${\gamma }_{\text{c}}$) and high transparency indicates the potential to achieve greatly enhanced modulation efficiency. In this study, a prototype PIN–PMN–PT phase modulator was fabricated based on a titanium (Ti) in-diffusion waveguide, which is reported for the first time. The influences of titanium in-diffusion on the composition and domain structure of the PIN–PMN–PT single crystal were studied by transmission electron microscopy (TEM) and piezoelectric force microscopy (PFM), respectively. Finally, a half-wave voltage (V_π) of 2.3 V was obtained using a device with 6-mm-long (L) electrodes. Furthermore, the electro-optic modulation efficiency (V_πL) was calculated as 1.38 V·cm, which was approximately one order of magnitude lower than that of commercial lithium niobate (LiNbO₃, LN) phase modulators. Such enhanced modulation efficiency indicates more compact device and lower power consumption, which is of great significance for electro-optic modulation systems used in micro-fiber gyroscope, integrated photonic devices, etc.