Aimed at the problem that vertical ram of a gantry milling machine is prone to flutter, which leads to the decrease of machining precision, an active vibration control method for multivariable gantry milling machine was studied. The modal of the vertical ram was analyzed, and the modal shapes of its structure were observed to study its dynamic characteristics.The transfer function was obtained by using the automatic power spectrum and the cross power spectrum in the MIMO system, and the frequency domain formula of the multivariable transfer function was obtained. The singular value information of the multivariable transfer function was used to obtain the mathematical model of the multivariable transfer system. The compensation function of the openloop system was obtained by using H∞ loop shaping method.On the basis of robust margin of disturbance system,the stability controller model was obtained to actively control vibration of the vertical ram of the gantry milling machine.The proposed method and PID method were used to control vibration of the vertical ram under different excitation signals.The results show that the maximum vibration amplitude of the vertical ram controlled by the proposed method is reduced by 39.88% and 37.21% respectively compared with that of PID method under square wave and sine wave excitation signals.The method has good effect on vibration control of vertical ram and can improve the stability of the gantry milling machine