Can you perform a coup de grace with a ray spell?
No. The coup de grace action specifically allows you to use
a bow or crossbow, provided you are adjacent to the target
(Player’s Handbook, page 154).
That said, allowing spells with a ranged touch attack to
deliver a coup de grace against an adjacent target wouldn’t be
an unreasonable house rule.
致命一击(Coup de Grace):你可以用近战武器以一个全回合动作对无助对手进行致命一击。你也可以用弓或弩进行致命一击,前提是你必须和目标相邻近。
根据FAQ对于ray进行cdg的解释,再根据致命一击的定义,好像只能用近战武器和弓或者弩,法术和其他远程武器应该示不行的。