Abstract: The control effect of attached flexible films on the torsional flutter of a single degree of freedom (SDOF) rectangular cylinder with aspect ratio B/D = 5 was studied through wind tunnel experiments. The influence of the attached flexible films with different parameters on vibration amplitude, surface wind pressure distribution and flutter derivatives of the tested rectangular cylinder was analyzed at the reduced wind speed U^* = 0.0~9.7. The test results show that the attached flexible films with moderate dimensionless stiffness K_B will flap. It can significantly increase the flutter critical wind speed of the rectangular cylinder, with a maximum increase of 93%. The suppression effect of the torsional flutter completely depends on the flexible films attached at the leading edges of the rectangular cylinder, not those at the trailing edges. The flapping flexible films reduce remarkably the mean and fluctuating pressure near the leading edges of the rectangular cylinder. The forced vibration result shows that the flutter derivatives A_2^* of uncontrolled case change from negative to positive, and enlarge with the increase of U^*. While the flapping films can maintain A_2^* approximately zero in the tested U^* range. It suggests the flapping films suppress effectively the negative aerodynamic damping caused by torsional motion, thus improving the flutter performance of the rectangular cylinder.