An AC system fault at the receiving end is the main cause of commutation failure in an Ultra High Voltage Direct Current (UHVDC) system. This can seriously affect the security of the power grid. To fully clarify the commutation process and control response of a valve group during commutation failure of a UHVDC system under an AC system fault, the actual extinction area and the minimum extinction area of the valve group are compared, and the main reason for commutation failure is analyzed through the electromagnetic transient simulation of the fault process. Sensitivity of operational parameters and commutation failure and the fault recovery process is studied. It is found that the parameters of a Voltage Dependent Current Order Limiter (VDCOL), proportional coefficient and integral coefficient of inverter control have obvious effects on commutation failure and the fault recovery process. These aspects can be the first choice for optimizing of the control system. This paper can provide a reference for commutation failure analysis and stable operation of a UHVDC converter station. This work is supported by the National Key Research and Development Program of China (No. 2016YFB0900600).