Commutation failure is one of the most common faults in the high-voltage direct current（HVDC） transmission system， which can lead to results such as: DC voltage reducing， DC current increasing， DC transmission power reducing， shorter life span of the converter valves， converter transformer DC magnetic bias and over-voltage of weak AC system on inverter-side. And in severe conditions， it may result in DC block fault and other serious problems. A DC block event occurred in the Gaoling back-to-back DC unitⅠ caused by AC system disturbance is described. AC system failure made DC system to produce commutation failure and to result in DC over-current protection action and unitⅠ block fault. Based on detailed event process， the principle of DC over-current protection action is discussed in detail， combining of Gaoling back-to-back DC system previous commutation failures. Finally， the recommendations and improvements about enhancing the security and stability level of AC and DC systems are given from four aspects such as: the DC system protection configuration， AC system stabilization level， DC control and protection system， and quality of power grid operating personnel.