Phase-matching is necessary to improve the efficiency of high-order harmonic generation. However, traditional phase-matching cannot be achieved in isotropic gas medium. As a feasible alternative, quasi-phase-matching breaks through the limitation of coherent length, which enables the harmonic intensity to continuously increase with the propagation distance. In this study, a quasi-CW counterpropagating terahertz field is used to modulate the generation of high-order harmonics to achieve quasi-phase-matching. The relationship between the harmonic spectra and the parameters of the modulation field is discussed through simulation. Results show that the quasi-phase-matched photon energy is inversely proportional to the wavelength of the terahertz modulation field and is directly proportional to the ratio of the field strength of the modulation to the pump field.