With the advantages of high automation， wide drilling adaptability， good mobility and high drilling efficiency， truck-mounted hydraulic drilling rig is widely used in coal bed methane extraction wells， water wells and so on. As an important component of the drilling rig， the mast supports the power head for rotary drilling. The vibrational stability of the mast has very important influence on the reliability of the drilling rig. Through modal analysis， the natural frequency and the modal amplitude of the mast are determined， and the results show that the resonant phenomenon may occur. Through the topological optimization analysis of the mast and according to the actual functions of the mast， the optimal model is obtained. The static structural analysis and the modal analysis results show that the optimization effect is remarkable， which meets the structural strength requirements and avoids the occurrence of resonance. The finite element analysis of the mast provides theoretical support for the structure design of the mast， and provides a modern mechanical structure design method.