The manned aerial vehicle with the telescopic arm is full of danger in the parameter tuning process. It is necessary to construct a vibration suppression controller which is effective and easy to tune. The telescopic arm was theoretically divided into two rigid bodies. The dynamics of the equivalent secondorder underactuated system was established based on the Lagrange equation. The model adopted the combination of elevation increment of the first link and tip acceleration of the second link to flatten the system, and made it into a secondorder single input single output model. The input gain and coefficient of zero-order term were automatically updated along with the elevation or length change of the arm and substituted them into the active disturbance rejection control (ADRC) system. Application of the proposed algorithm was shown on the simulation and experiment platform. The favorable results of arm stability was achieved under the controlling of flatness-ADRC in the case of changing in length and angle. Under the two working conditions of extension and shortening, the balance can all be restored within 2s. It was an innovation to design differential flat system with two different physical quantities, in addition the clear physical meaning of parameters in tuning process was very important in this liferelated system. Under the condition of simplified model, the parameters of the control system were adjusted according to the extension length and attitude, which ensured the system’s disturbance rejection ability to be continuous and effective.