From the principle of extinction angle control used in HVDC, the advantage and shortage of extinction angle control based on actual measuring and predictive calculation are contrasted. And the response strategies of commutation failure based on the Siemens and ABB technology are discussed from control and protection system. The response of commutation failure prediction control function (CFPREV) under AC voltage falling and distortion fault are focused. The results of simulation show that, when AC voltage falls seriously, the CFPREV cannot prevent commutation failure, but the CFPREV is in the favour of the restoration of DC system after commutation failure; when the electrical relation between DC and AC system is strong or the distortion degree of AC voltage is low, the CFPREV function has obvious effect on preventing commutation failure.