Abstract: A mass of water dropping for a time of large firefighting aircraft can produce dynamic response problem in flight. The variational process of water quality was deduced by applying hydromechanical simulation of fenestral outflow.Considering rigid-body modes and elastic modes, dynamic response analysis of water dropping for a firefighting aircraft was presented by applying minimum-state approximation formula of unsteady aerodynamics, in which the results of trim analysis were defined as the initial conditions. And the dynamic response characteristic of high pitch angle in exit flight was studied when the pilot input command was defined by elector deflection angle, in which the flying gesture after water dropping was the initial condition. Results show that flying gesture and loads of firefighting aircraft are slightly changed continuously, but they are changed obviously when in climbing exit flight with high angle. It provides future reference for the structural design and the pilot input command of large firefighting aircraft.