The invention discloses an online elastic mode testing system for magnetically suspended electromechanical equipment. The online elastic mode testing system mainly comprises a magnetically suspended system shell, a magnetically suspended rotor, a magnetically suspended control system, a signal excitation module, a vibration detection unit and an elastic mode recognizing module. The signal excitation module is integrated in the magnetically suspended control system, excitation force is applied to the rotor, unbalanced disturbance of translation and rotation and random environmental disturbance of the rotor are simulated, frequency sweeping excitation is increased under the gyroscopic action of a high-speed rotating rigid body, and output signals of a displacement sensor and the vibration detection unit are transmitted to the elastic mode recognizing module to obtain the translation and rotation elastic mode parameters of the rotor and the shell. By adopting the online elastic mode testing system, the influences of noise and unbalanced vibration on an elastic mode can be simulated close to the practical running state of a magnetically suspended, the frequency sweeping excitation output is increased, the near frequency discrimination of elastic modes is enhanced, a rotation elastic mode and a translation elastic mode can be distinguished, and the elastic modes of the rotor and the shell can be tested simultaneously.