This study presents a microwave photonic down-conversion system based on the stimulated Brillouin scattering. In this system, a local oscillator signal and radio frequency signal are introduced into two sub-Mach-Zehnder modulators of the dual-parallel Mach-Zehnder modulator to convert a high-frequency radio frequency signal to a intermediate frequency signal. Further, the gain spectrum of the stimulated Brillouin scattering effect is used to retain the two +1-order sidebands generated by the radio frequency and local oscillator signals. These two sidebands are then transmitted to a photodetector for beating, and an intermediate frequency signal is produced. Simultaneously, the phase of the intermediate frequency signal can be changed by adjusting the bias voltage of the parent Mach-Zehnder modulator in the dual-parallel Mach-Zehnder modulator. The frequency of the down-converted radio frequency signal is 10.73 GHz, which can be converted to a signal having any frequency in the range of 20-40 MHz and whose phase can be linearly transformed from 0° to 360°.