With the increase of the UHVDC capacity, the power fluctuations caused by commutation failures become more serious and probably induce out of step of AC grid which becomes an important external constraint condition of the commutation failure protection. This paper gets the ability of AC power grid to bear the number of DC power dropping and the fault duration time caused by commutation failures through DC commutation failure analysis firstly. Then the principle, criterion and action strategy of original commutation failure protection are introduced. Against the continuous commutation failures not meeting AC system tolerance capacity, an optimal scheme that adds double-pole commutation failure acceleration protection is proposed and the protection criterion, logic time sequence, setting values delay, and action strategy are given. Finally, the relevant software program of the proposed protection is written, and simulation experiments are done on the control and protection platform in line with real engineering which confirms the feasibility and reliability of the new scheme. The optimization scheme has been applied in the UHVDC transmission project from Yibin to Jinhua successfully.