Is it correct to use "actual self" in the following context:
As she was dead long time ago, her descendants could not have possibly seen her actual self.
There is nothing especially wrong with the term actual self, though to some extent, I am having to guess at what is means.
If you mean her real self, as opposed to her pretended or false self, then why not use real self ?
I think we need some wider context from you to understand exactly what you are getting at with actual self.
Incidentally the opening clause of your sentence I think needs to read As she had been dead a long time... or As she died a long time ago..., in order to be grammatical.