In　order　to　put　the　Internal　Stretch　Forming　Space　Lattice　Structure　(ISFSL)　into　practice,　based　on　the　Geometrical　Displacement-iterative　Method,　the　Displacement-iteration　Form-finding　Arithmetic　in　R　Coordinates　is　proposed,　in　which　only　the　displacement-iteration　is　exerted　on　span　coordinates　of　ISFSL.　A　detailed　study　on　the　form-finding　process　is　carried　out,　which　is　based　on　an　ISFSL　with　a　long　span　of　80　m.　The　result　shows　that　this　arithmetic　is　very　simple　with　great　efficiency　and　working　speed.　When　the　sixth　iteration　is　accomplished,　the　maximal　absolute　value　of　error　of　coordinates　is　only　as　follows:　ΔR=7.03E-04　m,　Δθ=6.7E-10,　ΔZ=1.04　E-01　m,　which　is　satisfied　to　the　engineering-oriented　need.　The　arithmetic　also　ensures　the　continuum　and　integrality　of　the　structural　forming　and　loading　analysis,　and　is　quite　fit　to　the　form-finding　study　of　ISFSL.