The cutter’s rotating deformation plays an important role which caused the vibration noise of grass trimmer. Highspeed video camera was applied to get the deformation images of grass trimmer’s cutter with protecting cover and without it. The tendencies and reasons of deformation were proposed. The finite element model of cutter based on experiment was established, the radial deformation was simulated by ANSYS, and the relations of the radial deformation between the cutter structure and rotational speed were analyzed. Reducing the area of big hole in the cutter could decrease the maximum radial deformation (in 12.000r/min, the value decreased by about 30%); Maintaining the motor speed at 8.000r/min around could control the maximum radial deformation below 0.2mm and keep a smooth moving. 