For permanent magnet servo system with large coefficient of viscous friction, traditional adaptive backstepping control could result in fluctuated and unstable identification results and poor performance of speed tracking. Adaptive backstepping controller identifying viscous friction coefficient was proposed, which was used to estimate the inertia, load torque and viscous friction coefficient simultaneously, improved parameter identification accuracy. The method proposed in this paper had merits of simple structure and usability. The validity and accuracy were verified by experiments based on the platform using dSPACE DS1103 system.