In the light of the door closing abnormal sound appears at the glass of the vehicle door in the full drop state, a method based on modal decomposition of operational deflection shape for identifying vibration sources of abnormal sound is proposed. First, time-frequency analysis of the noise signal is carried out through the short time Fourier transfer method. The result shows that the abnormal sound has secondary impacts. The energy of noise is mainly distributed in the low-frequency radiated section. Then, the test of operational deflection shape is used to identify door deformation. The modal frequencies and vibration shapes are obtained by modal test. Finally, the vibration source of the abnormal sound is diagnosed through the result of modal decomposition. According to this method, the main vibration source of the abnormal sound is identified successfully, and some suggestions for improvement are put forward. The improved test result shows that after the optimization, the secondary impacts and abnormal noise disappeared. The experimental result shows that the method is feasible and valid.