Taking CWGS250 roof rail type window cleaning machine as the research object, based on the principle of multi-degree of freedom free vibration, combined with the structure and bearing characteristics of the whole machine, the three-degree-of-freedom, two-degree-of-freedom and single-degree-of-freedom system dynamics of the whole machine are established by the centralized mass method. The model is obtained, and its natural frequency is obtained. The finite element software is used to analyze the whole machine model, and the natural frequency and mode shape of the first five modes are obtained, and the position of the larger displacement point and the relative displacement value are extracted. The results of the two methods are compared and analyzed. The two-degree-of-freedom system dynamics model can calculate the low-order frequency of the system more simply and provide a basis for further dynamic analysis.