Abstract: This paper aims to solve four-point constraint problems. To do that, this paper adopts nonlinear finite element method and Newton-Raphson incremental load-step method, and employs mobile boundary constraint conditions corresponding to the motion of mine-collecting machine. The space configuration of flexible pipes has firstly been computed under real boundary two-point constraints. Then it is modified by the displacement iteration method of sub-structure so that it matches the measured result under four-point constrains. Therefore the on-line dynamic measuring technique of the space orientation of flexible pipes for deep sea mining can be combined with the finite element computation.