When beat vibration is occurred in some kind of aeroengine, the vibration of the whole machinery will swing in some states. For this problem, the condition of its appearance is deduced. Combined with the characteristics of this kind of aeroengine, the reason and the location that the beat vibration came into being are studied through the analysis of vibration signals measured in different areas. Then，the synthesis of three vibration signals, the low pressu re rotor vibration, the high pressure rotor vibration, and the vibration of flexible rotor, is calculated. The method of solving the beat vibration problem is proposed by the numerical analysis. The results show that the ultimate reason of the vibration swing is the beat vibration appeared between the low pressure rotor and the flexible rotor. The effective way to avoid the vibration swing can decrease the vibration of low pressure rotor or the flexible rotor.