A　tomography　algorithm　is　proposed　for　the　reconstruction　of　the　three-dimensional　(3D)　distribution　of　the　real　refractive　index　of　a　pure　phase　sample　from　the　projections　of　cone-beam　propagation-based　imaging　(PBI).　The　algorithm　first　retrieves　the　integral　of　the　real　refractive　index　based　on　the　formula　described　in　this　letter,　and　then　a　conventional　computed　tomography　algorithm　is　performed　to　reconstruct　the　3D　distribution　of　the　real　refractive　index　of　the　sample.　The　computer　simulations　show　that　this　method　is　effective　for　PBI　tomography.