<p>For large precision aircraft structures, the main method of obtaining the dynamic characteristics of the structure is modal testing. The most important step in modal testing is to find a set of optimal excitation forces with appropriate distribution and amplitude to obtain the normal modal of the structure. Most of the existing methods for adjusting the excitation force rely on the experience of the researchers, with problems of time-consuming and insufficient accuracy in obtaining the modal parameters. This paper proposes a hybrid automatic force adjustment method in normal modal testing of structures which can solve the problem of force adjustment in dense mode. This method firstly measures the frequency response function of the structure, identifies the modal parameters through the polyreference least squares complex frequency domain method (PolyMAX), then use these modal parameters as the initial value of the normal modal test to derive the initial optimal excitation force. Finally, the optimal excitation force and the corresponding modal parameters are obtained by frequency sweep and particle swarm algorithm. Result shows that the method proposed in this paper can effectively solve the problem of local optimal and insufficient indicator function, and can reduce the force adjustment time of normal modal tests for large precision structures and improve the force adjustment accuracy.</p>