Photonic microwave harmonic downconverters (PMHDCs) based on self-oscillation optical frequency combs (OFCs) are interesting because of their broad bandwidth compared with plain optoelectronic oscillators. In this paper, a high-efficiency and flexible PHMDC is proposed and demonstrated. The properties of the OFC, such as the carrier-to-noise ratio (CNR), bandwidth, free spectral range (FSR) and the influence of optical injection are investigated. The broadband OFC provides a frequency tunable and high-quality local oscillation (LO), which guarantees flexible down conversion for the RF signal. The sideband selective amplification (SSA) effect not only improves the conversion efficiency, but also promotes single-sideband modulation. The conversion range can reach 100 GHz. The 12-40 GHz RF signal can be downconverted to IF signals with a high conversion efficiency of 14.9 dB. The fixed 40-GHz RF signal is flexibly down converted to IF signal with the frequency from 55.4 MHz to 2129.4 MHz. The phase noise of IF signal at a frequency offset of 10-kHz is the same as that of the input RF signal. The PMHDC shows great performance and will find applications in RoF, electronic warfare receivers, avionics, and wireless communication systems.