这句话怎么办?其中,that可不可以用who替代?After years' research the journalist found the only survivor that witnessed the disaster.

不可以，
一般情况下先行词是人在从句中作主语，可以用that或者who
但是这个句子先行词前有the only修饰只能用that

什么叫做这句话怎么办?怎么翻吗?
"经过多年的调查,记者终于找到当年这场灾难中的唯一幸存者"
不能,因为有only限制了,以下是在定语从句中只能用that的情况.
在定语从句中的运用.
1． 当先行词是不定代词all,everything,little, anything,nothing,much,few 等词时关系代词只用that.
e.g. There is something else that you must pay attention to.
2． 当指物的先行词被 all ,every , any, only, the very, last 等修饰时,关系代词用that.
e.g.This is the only problem that I can’t answer.
3． 当先行词被形容词*或序数词修饰时,用that.
e.g. He is the first student that came on time.
4． 先行词为人和物时,用 that.
e.g. We talked about the persons and things that we could remember at school.
5． 以who,which,what 开头的疑问句为避免重复用that.
e.g. Who is the person that you want to talk with?
现在明白了吗?要把英语学精任重道远,加油哈