Thorium　oxide　has　many　important　applications　in　industry.　In　this　article,　theoretical　calculations　have　been　carried　out　to　explore　the　hydrolysis　reactions　of　the　ThOn　(n=1–3)　clusters.　The　reaction　mechanisms　of　the　O-deficient　ThO　and　the　O-rich　ThO3　are　compared　with　the　stoichiometric　ThO2.　The　theoretical　results　show　good　agreement　with　the　prior　experiments.　It　is　shown　that　the　hydrolysis　mainly　occurred　on　the　singlet　potential　surface.　The　overall　reactions　consist　of　two　hydrolysis　steps　which　are　all　favourable　in　energy.　The　effects　of　oxygen　content　on　the　hydrolysis　are　elucidated.　Interestingly,　among　them,　the　peroxo　group　O22−　in　ThO3　is　converted　to　the　HOO−　ligand,　behaving　like　the　terminal　O2−　in　the　hydrolysis　which　is　transformed　into　the　HO−　groups.　In　addition,　natural　bond　orbital　(NBO)　analyses　were　employed　to　further　understand　the　bonding　of　the　pertinent　species　and　to　interpret　the　differences　in　hydrolysis.　©　2022　Wiley-VCH　GmbH.