Rhimes told TV Line that she normally figures out the season finale first but explained that she has been unable to do so this year because of the uncertainty surrounding the show's cast members.
The contracts for stars including Patrick Dempsey, Justin Chambers, Ellen Pompeo and Sandra Oh will expire in May, meaning that they may decide to leave the show.
Dempsey and Pompeo have both already sparked rumours that they will not return for a potential ninth season.
Rhimes has now admitted that the uncertainty has affected her planning, saying: "I don't know what I'm going to do at the end of the season yet. Until we know [who's coming back], we just don't know [how the season will end]. So we're taking it in a different way."
She continued: "We're looking at it in segments. We're planning the first half of the season [at the moment] and then I'll plan the second half of the season when I know more stuff."
Rhimes, who has previously insisted that Grey's Anatomy will not end after this season, remained confident about the show's future.
However, she admitted that she doesn't know who will return next year, saying: "I have no idea. That's not my job."
The new season of Grey's Anatomy will begin with a two-hour premiere on September 22 on ABC.
> Grey's Anatomy to feature 'all boys' episode
> Read more Grey's Anatomy coverage
Watch a trailer for the new season of Grey's Anatomy below: