In the dynamic test system of PMSM, the tested machine, dynamic torque meter and load machine are connected in a coaxial rotating machinery by two couplings. More than 70% of mechanical faults are caused by misalignment. The frequency response of parallel misalignment in a PMSM test bench was studied in this paper. A motion equation of shafting based on Lagrange method is established. The frequency response of the shafting with parallel misalignment was analyzed by the numerical method.