Abstract: Based on the mechanical and magnetic structure near the movable gripper coil of magnetic type control rod drive mechanism, a simplified simulation mechanism with size ratio of 1:2 was set up. Based on this system, motion characteristics of the movable armature were studied. Both the magnetic circuit calculation and ANSYS finite element method were used to theoretically analyze the factors on critical current of movable gripper coil in lifting and releasing process. The results compared with theoretical analysis and experimental test showed that ANSYS FEM results had better coherence with the measured values. ANSYS FEM results showed that the releasing current increased with reducing the permeability of the non-magnetic shim and increasing the thickness of the non-magnetic shim. Other factors including permeability of ferromagnetic materials, lifting load and the lifting gap also had a significant influence upon the critical lifting current and/or the critical releasing current.