'Nailin Palin' Star Lisa Ann -- Another Porn Star Is ALL OVER ME ... And Not In a Good Way!

Two legendary porn stars going toe-to-toe is usually a good thing -- artistically speaking -- but in this case Lisa Ann says fellow actress Nikki Benz is threatening her life ... so she's filed a restraining order.

In the docs, filed Friday in LA. ... Lisa says Nikki began attacking her on Twitter last December, and incited her more than 700,000 followers to do the same. Lisa claims Nikki was "encouraging and rewarding" those who made threats on her business and her safety.

Lisa -- who recently retired from the biz -- says the threats continued into January ... including someone threatening to rape and kill her unless she deposited money into an account. She says the threats became so violent and frequent ... she couldn't go to the AVN Awards to accept her Lifetime Achievement Award.

Lisa and Nikki have done some fine work together in the past -- for example, "Reservoir Sluts" and "Big Rack Attack 8" -- and it's unclear what sparked the feud.

Nikki tells us she has never threatened Lisa, and claims Lisa was actually harassing her on Twitter.