I killed a process by process ID accidentally, but now I need to know the process name.I checked the id number under folder /proc,but I got anything. how can I know the process name please.
There is no standard mechanism in Linux to discover the
name of a deleted process.
While there is no map of every pid used or log of all of them any logs to journald contain a pid so if it was a daemon you can check for it with:
sudo journalctl _PID=[PID]
note that this only checks logs for the current boot.