We demonstrate, to the best of our knowledge, a novel generation process of a terahertz (THz) electromagnetic wave via a noncollinear ${\chi }^{\left(2\right)}$ process in $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ with a phase-front-controlled femtosecond Ti:sapphire laser pulse. Systematic measurements of both the THz electric field and the excitation transmitted through an $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ crystal at different excitation powers show that the maximum amplitude of the THz electric field increases nonlinearly above the critical input powers. This enhancement of optical rectification efficiency is attributed to the distortion of the excitation pulse shape via electro-optic (EO) phase modulation by the emitted THz wave. Actually, spectral line shapes of both the THz wave and the excitation pulse become broadened above the threshold, and they are well reproduced by numerical simulations with equivalent one-dimensionally mapped coupled equations in the frequency domain. This generation scheme shows potential optimization of further generation efficiency.