卸磨杀驴

xiè mò shā lǘ

lit. to kill the donkey when the grinding is done (idiom)

fig. to get rid of sb once he has ceased to be useful

卸

xiè

to unload

磨

mò

grindstone

杀

shā

to kill; to slay; to murder; to attack

驴

lǘ

donkey