Modal analysis is the vital prerequisite of the full-scale fatigue test of the wind turbine blade. In order to resolve the difficulty in solving the characteristic equation of the transfer matrix method in modal analysis, a method of solving the characteristic equation based on particle swarm optimization is proposed. Firstly, the beam model is used to simplify and discrete the blade along the spanwise of the blade. A concept of average bending stiffness is used for the elasticity assignment with beam segment, and the blade kinematics model is established. Subsequently, a designed particle swarm optimization is used to solve the characteristic equation. Finally, the proposed method is verified by modal analysis of an MW-scale blade. On this basis, the effect of the mass and position of the counterweight on the mode shape is investigated. The results indicate that the proposed method can provide a theoretical reference for the fatigue test technology of wind turbine blades.