The deblocking and starting of HVDC converter is a common and basic operating mode. Different applications of SCR converter lead to various unique deblocking methods. Therefore, it is necessary to study the principle of deblocking and starting. Combining the equipment parameters of the actual engineering and using the electrical circuit theories and power electronic technologies, the triggering of the converter single valve is approximated as the transient process of the second-order RLC circuit response. The process of capacitor charging of RC circuit of other valves is analyzed when a single valve is turned on, and the law of bias voltage change is deduced. The formation mechanism of the full bridge conduction of converter is studied, the criterion of full bridge conduction of the12-pulse bridge and the the expression of output pulse voltage are given to explain the process of continuous current generated by the high-voltage DC system to successfully deblock. The electrical recording data obtained during deblocking the inverter during an actual project commissioning verify the correctness of theoretical derivation of this paper.