According to the differential geometry theory and counter-analysis principle

this paper deduces the spiral curve arc-length base equation of higher degree from its total construction.Applying the theory about the modern mathematics and the survey adjustment

it analyses the astringent properties in the arc-length base equation of the spiral curve and advances a method—the iterative method to get the arc-length of the spiral curve directly and precisely.Applying the technologyical project on "p substitute for A"

making a breakthrough at the old and trouble method—"parameter estimate method "0

getting the arc-length of the spiral curve

the new technological project is used in the highway.There are some conclusions in this paper: the history that no arc-length base equation for the spiral curves has been closed;it is important that the special characteristics of the arc-length base equation may be employed universally in the curve composing and positioning.