The dynamic characteristic design is a key issue in helicopter design. Good dynamic characteristics of the airframe will avoid or significantly reduce the cost of vibration reduction in the later stage. In this paper, a whole dynamic model of a helicopter is established by adopting the modular approach. At the same time, tests are also designed to verify the calculated modal properties and vibration response values of the helicopter airframe. Based on the test data, the dynamic model is corrected using the modal assurance criteria and trial-and-error method. In response to the problem of abnormal lateral vibration occurred on the vertical tail of a helicopter during the research flight test, the corrected model is used to determine that the cause is the resonance between the airframe lateral bending mode frequency and the main rotor passing frequency. A solution of adding counterweights is provided to ensure the flight progress with minimal structural changes. A lateral dynamic absorber is ulteriorly designed. Compared to the adding counterweight scheme, the absorber has a lighter additional mass, better vibration reduction effect and comparable installation difficulty making it a more excellent vibration reduction scheme.