When a coherent beam transmits by a multimode fiber,speckles will appear in the output field.Vibrating the fiber is approved to be an effective way to suppress the speckles.To reveal the relationship between the vibration parameters and the suppression effect,we establish the formula connecting the amplitude,frequency,acting length to the speckle improvement based on the rate of mode coupling.The experiment results confirm the trend:the suppression effect improving with vibration amplitude,and changing nonlinearly with the frequency、acting length.Therefore the rate of the mode coupling can evaluate the suppression effect qualitatively.The results provide theoretical proofs and experimental reference.