The phase-conjugate ultrafast modulation spectroscopy is studied theoretically due to the interference between fifth-order optical polarization in a V-type three-level system, and considered the cases that the pump beams have either narrowband or broadband linewidth. It is found that the accuracy of the energy-level splitting measurement is determined by the homogeneous linewidths of the optical transitions. It means that this technique can achieve Doppler-free precision in the measurement of energy level splitting.