Considering the short circuit fault at DC(direct-current) converter side, the rectifier generator with DC converter can be equivalent to the rectifier generator suddenly applied with RLC(resistance inductance capacitance) load, and its transient current calculation is directly related to DC fault diagnosis and protection. When the RLC load was equivalent from the DC side to the AC(alternating-current) side, the transient current was equivalent to the three-phase short circuit current under the condition of the synchronous generator in series of RLC. Through the expression of the transient current, the analytical expression of the transient current was obtained by calculating the initial value, final value and time constant with the method of analogizing the three-phase short-circuit current′s calculating process of the synchronous generator, and the peak value and peak time of the current were given. Finally, the validity of the analytical expression is verified by simulation,which can be used for the improvement of the diagnosis and the determination of the protection way of DC faults.