The framework is the main load bearing part of beam pumping unit. Its vibration performance has an important effect on the stability and reliability of beam pumping unit. This article takes the beam pumping unit (API C-228D-173-74) as an example, carries out the modal analysis in the environment of Solidworks Simulation, and obtains the first 6 orders of modal shapes and natural frequencies of the prestressed framework. This article clarifies the key technology of how to build the solid modal and how to apply load and constraints. The analysis result of the modal shows that the lowest frequency of the pumping unit framework (API C-228D-173-74) is 22.39Hz, which may be most likely resonant with the motor. We can adjust the rotate speed of the motor to avoid resonance. At the same time, the stiffness of the front legs is insufficient, suitable strengthen measures should be taken.