Theranostic　nanomedicines　that　integrate　diagnostic　and　therapeutic　moieties　into　a　single　nanoscale　platform　are　playing　an　increasingly　important　role　in　fighting　cancer.　Here,　a　facile　and　green　synthetic　strategy　for　hollow　CoPt　alloy　nanoparticles　(HCPA-NPs)　using　plant　polyphenols　as　assisted　agents　is　reported　for　the　first　time.　This　novel　strategy　enables　size-controlled　synthesis　of　HCPA-NPs　through　the　control　of　the　molecular　sizes　of　polyphenols.　It　is　also　a　versatile　strategy　for　synthesizing　other　hollow　alloy　nanoparticles　with　various　metal　compositions　due　to　the　diverse　metal-chelating　ability　of　the　polyphenols.　Further　studies　show　that　HCPA-NPs　have　good　biocompatibility　and　can　be　successfully　implemented　for　magnetic　resonance　and　photoacoustic　dual-modal　imaging　guided　photothermal　therapy.　This　work　brings　new　insights　for　the　green　synthesis　of　hollow　nanoparticles　and　extends　these　biocompatible　nanoparticles　for　theranostic　applications.