Abstract: Applying the theory of finite deformation and the experimental strain analysis by the Moire method, the two dimentional displacement field and strain field in a plane model of slush pump rubber piston is investigated to determine the strain and displacement of the piston under different pressure and to depict the changing tendency of the fields. On the basis of experimental results, the geometrical and physical characteristics of some common pistons under static state are discussed and the mechanics of nylon MC ring in protecting the heel from wear explained. Besides applying the principle of finite deformation to tile study of rubber deformation, the author has also attemptel to tackle sealing problems with the Moire method.