The bipolar short-circuit fault at DC line of MMC-HVDC transmission system is a serious threat to the safe and stable operation of the system. The transient process of the fault is studied and the fault characteristics are analyzed in detail. Firstly, an equivalent circuit model of MMC sub-module dynamic casting process is put forward for the pre-blocking stage of the converter. The simulation results show that the equivalent circuit model can accurately and effectively reflect the high-frequency switching process of the MMC sub-module. Alternating discharge of the sub-module capacitance makes the DC-side fault current rise more quickly. Secondly, a mathematical description equation is established with the equivalent circuit for the post-blocking stage of the converter, and a three-stage analysis method is proposed for the post-blocking stage of the converter. The analysis shows that after the inverter is blocked, in the currents of each bridge arm, there is a non-periodic component of the bridge arm inductance at the moment of blocking in addition to the non-periodic component of the capacitor discharge before the blocking; when the DC-side outlet fails, the peak value of the fault current may appear after the inverter is blocked. PSCAD/EMTDC model simulation verified that the theoretical analysis result is correct.