BaTiO3 crystal thin film has been investigated to realize electro-optic (EO) devices due to its ultrahigh EO effect, but not much research has been focused on the correct axis orientation and EO coefficient. In this Letter, with a BaTiO3 crystal film grown by pulse laser deposition technique on ⟨100⟩ MgO crystal substrate, an embedded device configuration having the two-step etched waveguide/electrode scheme is designed to reach a high optic-electrical field interaction efficiency, and a 45° outside electric field with an in-plane axis is set to fit the possibility of the a-axis or c-axis orientation of the BaTiO3 crystal film. Then, through a poling process to the sample, the 1π, 2π, and 3π EO modulations of linear polarization are implemented at 4.9, 9.3, and 11.8 V, respectively, and a decremental voltage period is found. Thereby, a c-axis oriented BaTiO3 crystal film is determined, so that the nonlinear modulation equation is exploited. Finally, with an overlap of 1π, 2π, and 3π modulations, the coherent EO coefficient r51 and birefringence of 606 pm/V and -0.0215 are obtained, resulting in one (Vπ)2L value of 68 (V2·mm).