Focusing　on　distributed　antenna　systems,　we　propose　a　photonics-assisted　self-interference　cancellation　and　down-conversion　method　based　on　a　Sagnac　loop　structure.　Two　inversely　cascaded　modulators　form　a　Sagnac　loop　through　polarization　multiplexing.　To　support　in-band　full　duplex　(IBFD)　communication,　a　double　parallel　Mach-Zehnder　modulator　(DPMZM)　operated　in　a　clockwise　direction　is　used　as　a　self-interference　canceller　(SIC)　to　achieve　the　phase　conversion　in　the　optical　domain.　Another　MZM　modulator　working　in　the　counterclockwise　direction　is　used　to　modulate　the　local　oscillation　(LO)　signal　for　frequency　down-conversion.　The　adjustment　of　bias　voltage　is　analyzed　to　optimize　the　conversion　efficiency　of　the　system.　The　power　fading　effect　generated　by　fiber　dispersion　is　compensated　by　a　polarization-sensitive　phase　modulator　(PS-PM).　The　experimental　results　show　that　the　average　cancellation　depth　is　56.4　dB　at　a　single　frequency　in　the　operating　frequency　range　of　6-22　GHz,　and　29.1　dB　and　23.7　dB　for　20　MHz　and　100　MHz　bandwidth　signals,　respectively.　In　addition,　the　error　vector　map　is　improved　from　16.50%　to　6.26%　after　dispersion　compensation.　The　self-interference　cancellation,　down-conversion　and　dispersion　compensation　are　integrated　in　a　microwave　photonics　link.　Two　cascaded　modulators　are　equivalent　to　a　parallel　structure　by　the　Sagnac　loop,　simultaneously　ensuring　high　LO　isolation　and　conversion　efficiency.　The　proposed　method　provides　an　alternative　for　IBFD-based　distributed　antenna　communication　applications.