Cloud　storage　services　offer　flexible,　convenient　solutions　for　business　and　personal　users　to　store　data.　Traditionally,　Third　Party　Auditors　(TPAs)　are　introduced　to　ensure　data　integrity　for　public　auditing.　However,　TPAs　may　also　be　untrusted　for　forging　the　auditing　results　or　colluding　with　cloud　storage　servers　to　deceive　users.　In　this　paper,　we　propose　a　novel　Blockchain-based　Public　Auditing　Outsourcing　system　without　TPAs　(BPAO),　in　which　the　computationally　expensive　operations　in　public　auditing　are　outsourced　through　blockchain　to　the　cloud　servers　without　risking　users＇　privacy.　Our　security　analysis　indicates　that　BPAO　achieves　soundness　and　robustness.　The　experimental　results　show　that　BPAO　is　computationally　efficient　for　cloud　storage　user.　©　2021　IEEE.