A new technique of using auxiliary-mirror holography to analyse 3D vibration is proposed. The intensity distribution of the auxiliary-mirror time-averaged holographic interferogram is discussed. The solution in which the above holographic technique is used to measure the 3D amplitude and the method which draws information of the virtual point sources of the refcrence beaxns with the help of quasi-same-position auxiliary-object holography are explained. The problem of the position of the zero order bright fringe of the auxiliary-mirror time-averaged holographic interferogram is analysed in theory.