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 80m. 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-04m, Δθ=6.7E-0°, ΔZ=.04 E-0m，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. 