Abstract: STACER is a kind of widely used deployable mechanism on spacecraft, whose thin-wall steel strip is scrolled and eventually formed a spiral tube. Because the compressed tube can be extended dozens of times of its initial height; large deflection with large strain appears, and the deformation scope of the strip are continuously changed during the deployment, the deployment force is extremely di cult to solve even for numerical simulation. Here, using the variational method and the geometric characteristics of the deformation, a technique is proposed to solve the deployment force:at first the proper and possible shape function is constructed, and then the principal strain and the deformation energy are calculated by means of the shape function,finally the deployment force is carried out through the principle of minimum potential energy. This method makes the numerical procedure simplified and gives approached expression of the strip shape and the deployment force.