To achieve the structural stability of the mast of rotary drilling rig in the course of construction, analysis theory and method of FEM is applied to the study of the mast deformation of rotary drilling rig. Firstly, three-dimensional models of the mast of rotary drilling rig are created by the software of Pro/E, and then form a corresponding finite element model to carry out an analysis on the bending deformation of the mast of rotary drilling rig by the software of Ansys and to disclose the deformation regularity of the mast. The corresponding test is subsequently performed to obtain actual deformation amount of the mast of rotary drilling rig. Finally, the deformation results from the test conform to that from the software of Ansys. The method and experience of this paper will provide theoretic supports for more optimization design and research of the mast of rotary drilling rig.