We　propose　and　demonstrate　a　highly　efficient　photonic-assisted　microwave　signal　harmonic　frequency　down-　conversion　using　four-wave　mixing　(FWM)　in　a　silicon　waveguide　with　a　reverse-biased　P-i-N　junction.　The　FWM　phenomenon　occurred　in　the　integrated　waveguide　due　to　the　doped　regions　of　the　junction　can　increase　the　efficiency　of　free　carrier　removal　from　the　waveguide　region.　Correspondingly,　the　conversion　efficiency　is　up　to　12.8　dB　with　the　bias　voltage　of　30　V.　The　silicon　waveguide　is　fully　packaged　with　a　grating　coupling　fiber　array　and　a　wire-bonded　printed　circuit　board　(PCB).　By　simply　adjusting　the　low-frequency　electrical　LO,　the　down　converter　based　on　FWM　can　realize　accurate　tunable　and　ultra-wide　frequency　operation.　This　scheme　can　significantly　reduce　the　high-frequency　demand　for　LO　source.　In　the　experiment,　the　RF　signals　ranging　from　8　GHz　to　31　GHz　are　all　down　converted　within　2　GHz　by　utilizing　the　5.88.2　GHz　LO　signals,　and　signal-to-noise　ratios　(SNRs)　larger　than　50　dB　are　achieved.　Moreover,　the　phase　noise　of　the　intermediate　frequency　(IF)　signal　is　as　low　as　-80.12　dBc/Hz　at　an　offset　frequency　of　10　kHz　and　spurious　free　dynamic　range　(SFDR)　is　105.5　dB　center　dot　Hz(2/3)　with　3rd-order　FWM　effect　for　the　LO　signal　and　103.8　dB　center　dot　Hz(2/3)　with　4th-order　FWM　effect　for　the　LO　signal.　This　shows　that　the　down　conversion　signal　is　reliable　to　further　use　in　modern　electrical　systems　such　as　microwave　photonic　radar.