Background:　The　COVID-19　infodemic　has　been　disseminating　rapidly　on　social　media　and　posing　a　significant　threat　to　people＇s　health　and　governance　systems.　Objective:　This　study　aimed　to　investigate　and　analyze　posts　related　to　COVID-19　misinformation　on　major　Chinese　social　media　platforms　in　order　to　characterize　the　COVID-19　infodemic.　Methods:　We　collected　posts　related　to　COVID-19　misinformation　published　on　major　Chinese　social　media　platforms　from　January　20　to　May　28,　2020,　by　using　PythonToolkit.　We　used　content　analysis　to　identify　the　quantity　and　source　of　prevalent　posts　and　topic　modeling　to　cluster　themes　related　to　the　COVID-19　infodemic.　Furthermore,　we　explored　the　quantity,　sources,　and　theme　characteristics　of　the　COVID-19　infodemic　over　time.　Results:　The　daily　number　of　social　media　posts　related　to　the　COVID-19　infodemic　was　positively　correlated　with　the　daily　number　of　newly　confirmed　(r=0.672,　P＜.01)　and　newly　suspected　(r=0.497,　P＜.01)　COVID-19　cases.　The　COVID-19　infodemic　showed　a　characteristic　of　gradual　progress,　which　can　be　divided　into　5　stages:　incubation,　outbreak,　stalemate,　control,　and　recovery.　The　sources　of　the　COVID-19　infodemic　can　be　divided　into　5　types:　chat　platforms　(1100/2745,　40.07%),　video-sharing　platforms　(642/2745,　23.39%),　news-sharing　platforms　(607/2745,　22.11%),　health　care　platforms　(239/2745,　8.71%),　and　Q&A　platforms　(157/2745,　5.72%),　which　slightly　differed　at　each　stage.　The　themes　related　to　the　COVID-19　infodemic　were　clustered　into　8　categories:　＂conspiracy　theories＂　(648/2745,　23.61%),　＂government　response＂　(544/2745,　19.82%),　＂prevention　action＂　(411/2745,　14.97%),　＂new　cases＂　(365/2745,　13.30%),　＂transmission　routes＂　(244/2745,　8.89%),　＂origin　and　nomenclature＂　(228/2745,　8.30%),　＂vaccines　and　medicines＂　(154/2745,　5.61%),　and　＂symptoms　and　detection＂　(151/2745,　5.50%),　which　were　prominently　diverse　at　different　stages.　Additionally,　the　COVID-19　infodemic　showed　the　characteristic　of　repeated　fluctuations.　Conclusions:　Our　study　found　that　the　COVID-19　infodemic　on　Chinese　social　media　was　characterized　by　gradual　progress,　videoization,　and　repeated　fluctuations.　Furthermore,　our　findings　suggest　that　the　COVID-19　infodemic　is　paralleled　to　the　propagation　of　the　COVID-19　epidemic.　We　have　tracked　the　COVID-19　infodemic　across　Chinese　social　media,　providing　critical　new　insights　into　the　characteristics　of　the　infodemic　and　pointing　out　opportunities　for　preventing　and　controlling　the　COVID-19　infodemic.