The invention relates to a device for measuring dynamic stiffness of a complete machine in the state of rotation of a spindle of a milling machine. The device comprises a base, an electromagnetic excitation mechanism, a simulation tool, a tool handle and a measuring mechanism. The electromagnetic excitation mechanism comprises a frequency generator, an alternating power supply and an electromagnetic exciter, wherein the frequency generator, the alternating power supply and the electromagnetic exciter are electrically connected in sequence. The electromagnetic exciter is horizontally supported on the base, and the axis extension line of the electromagnetic exciter and the axis line of the simulation tool are orthogonal. The simulation tool comprises a permanent magnet core, a stepped-shaft-shaped magnetism isolation bar and a sleeve, wherein a through hole is formed in the permanent magnet core in the axial direction, one end of the stepped-shaft-shaped magnetism isolation bar is inserted in the permanent magnet core, and the other end of the permanent magnet core and the other end of the magnetism isolation bar are sleeved with the sleeve. The measuring mechanism comprises a force plate, three eddy-current transducers and two laser displacement sensors, wherein the force plate is fastened and connected to the bottom of the base and is used for measuring the dynamic force of the electromagnetic exciter, the three eddy-current transducers are used for measuring the X radial dynamic displacement, the Y radial dynamic displacement and the end face dynamic displacement of the tool handle respectively, and the two laser displacement sensors are used for measuring the X-directional dynamic displacement and the Y-directional dynamic displacement of the simulation tool relative to the base respectively.