The mast of coal bed methane truck-mounted rig is an important executive device， while the mast support frame is significant for supporting the mast， and its vibration stability determines the reliability of the mast. The finite element modal method is used to analyze the support frame under both the free vibration state and the prestressed state， and the numerical results and the change rule of the natural frequency and the modal shape are attained for the two states. Comparative analysis shows that the results for the two state are close but with slight difference， indicating that prestress is of little impact on vibration. At the same time， it is verified that the mast support frame will not generate resonance. Moreover， the modal results provide a theoretical support for the structural design of the frame， and a theoretical basis for topology design and in-depth dynamic analysis with crucial significance.