Law enforcement sources tell us ... 20-year-old Musso was pulled over at 3:43 AM Sunday ... after he failed to slow down for cops who were directing traffic for an unrelated accident.

We're told ... cops approached Musso's 2007 black Mercedes Benz and detected a "strong odor of alcohol."

Musso -- who also stars on the Disney hits "Pair of Kings" and "PrankStars" -- was given a field sobriety test ... and we're told he performed poorly.

Law enforcement sources also tell us ... Musso was given a breathalyzer test and blew WELL ABOVE .08 ... the legal limit in CA for people who are OVER the age of 21.

Musso was booked on suspicion of DUI and hauled to a nearby station. Bail was set at $5,000.