11:47 AM PT -- Unser was so drunk, he couldn't stand up straight and actually fell down an embankment during the arrest ... this according to court docs obtained by TMZ Sports.

The responding officer says he initially pulled Unser over when he saw the racing legend whiz by him in a black Volkswagen Beetle around 1:12 AM.

During the stop, the officer says Unser reeked of booze and couldn't keep his balance ... and the entire time Unser insisted he hadn't had anything to drink before getting behind the wheel.

At one point, the officer says ... "Unser Jr. walked onto the embankment, lost his balance, fell to the ground and rolled down the embankment."

Unser refused breath tests and other field sobriety tests so he was cuffed and taken to the station for a blood draw.

The officer says Unser was a pain in the ass during the entire ordeal -- "he was consistently verbally hostile" toward the cops involved in the situation.

No word on the results of the blood draw ... yet.

Well, this is bad ...

Racing legend Al Unser Jr. -- who won the Indy 500 TWICE -- was arrested for drunk driving in Indianapolis early Monday morning, officials say.

The 57-year-old was pulled over in Hendricks County -- and officers suspected he was drunk. He was hauled to a nearby station where Unser was booked around 3:20 AM.

Court records show he's facing 3 charges -- improper lane usage, speeding and operating while intoxicated.

If Unser doesn't seem fazed in his mug shot ... it's because it's the 3RD TIME he's been arrested for drunk driving.

He was previously popped for drunk driving in 2007 and 2011.

Obviously, Unser struggles with alcohol abuse. He was also arrested in 2002 after a booze-fueled altercation with his then-girlfriend.

Unser has spoken publicly about his issues and has tried to get help multiple times.

Unser retired from racing back in 2007 after a stellar career which includes an Indy 500 win in 1992 and again in 1994.