Sound field in a car’s irregular enclosed cavity under structural excitation was researched by the advanced Trefftz analytic method. Combining with the acoustic structure coupling and using a weighted residual formulation to deal with the boundary condition, the analytical solution expression of the progressional expanded wave function was obtained, and the prediction analysis solution at mid-low frequency was given. The active noise control model for the cavity was established. The feasibility of the method was proved by mathematical simulation result using Matlab.