Terahertz frequency multiplier based on high-order harmonic generation of GaAs-based miniband superlattices is studied by using balance equation approach. With the presence of magnetic field, the peak power output of the frequency multiplier and its distribution in the parameter space (E_dc, E_ac) are carefully analyzed. The study shows that in the parameter plane of (E_dc, E_ac), the magnetic field has little influence on the peak size of the second and third harmonic of output power, but the magnetic field will broaden the peak area of the harmonic output power, which can improve the probability of output peak power in the parameter space of (E_dc, E_ac). When E_ac is determined, the position of the peak harmonic emission power will be affected by the Bloch oscillation frequency f_B generated by the DC electric field, the modulated Bloch oscillation frequency f_MB generated by the AC electric field, and the cyclotron oscillation frequency f_c caused by the magnetic field. The results indicate that the THz frequency multiplier based on semiconductor superlattice is a potential THz wave generator.