Abstract: 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 unitⅠcaused 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.