The bearing model and rotor model are firstly built in this paper. The connection relationships between the housing, bearing, shaft and tool of highspeed motorized spindle are analyzed. A dynamical model of highspeed motorized spindle is established, and the transfer functions of different parts of the motorized spindle to the tool are deduced. Then, based on the D62D24A type motorized spindle, the weakening effect of speed on bearing dynamical stiffness is analyzed. The first order inherent modal shape and inherent frequency are investigated numerically and experimentally for both situations of with and without the tool. The results show that both tool and speed have a “softening” effect on the inherent characteristics of system.