Limited by the output displacement, the Giant Magnetostrictive Actuator (GMA) cannot drive the spool directly. A hydraulic displacement amplifier based on flexible piston is designed; the flexible piston is pushed by the deformation of Giant Magnetostrictive Material (GMM), and finally pushed the small piston with much larger output. The coupled model of hydraulically amplified GMA is established; the structure is analyzed based on elastic small deformation theory and finite element method (FEM); and through simulation and test of a prototype, the output displacement is amplified 3.2 times by hydraulic amplifier. The proposed model is valid with an error beneath 10%.