Flutter performance of PK section girders for long-span bridges in the finished bridge state was investigated based on a cable-stayed bridge with the main span of 820 meters by means of sectional models’ wind tunnel tests and the 2D-3DOF method. A comprehensive criterion with root mean square (RMS) deviation, peak factor and damping ratio of torsional responses of girders under critical wind speed to determine the critical point of “soft flutter”was proposed. The flutter control effect and flutter drive mechanism for 3 sizes of airflow-suppressing lamina were explored. The study showed that PK section girders in the finished bridge state have obvious “soft flutter” characteristics, the effects of wind attack angle are also obvious, especially, the flutter critical wind speeds under the wind attack angle of 0° and 3° have a remarkable difference, girders reveal bending-torsion coupled flutter under the wind attack angle of 0, while they reveal single–DOF torsion flutter under the wind attack angle of 3 due to their obviously different aerodynamic damping variation laws; airflow-suppressing lamina can effectively improve the flutter critical wind speed of PK section girders under the wind attack angle of 3° to increase flutter coupling level; although more aerodynamic damping going against coupling are excited with airflow-suppressing laminas installed, the aerodynamic damping generated by torsional motion can enhance the system’s stability; the competition among different types of aerodynamic damping determines if the system diverges.