Skai Jackson won't face charges after her domestic battery arrest after fighting with her BF ... 'cause prosecutors say there just isn’t enough evidence to bring the case to trial.

The L.A. County District Attorney's Office tells TMZ ... they’re declining to file charges because Skai has no criminal history, the alleged victim lacks injuries, and he's refused to cooperate with prosecutors.

TMZ broke the story of the Disney Channel alum’s arrest ... after cops say she was spotted getting into it with her BF last week at Universal CityWalk.

We're told officers responded to the scene at CityWalk and security detained the couple after video showed Skai pushing her BF.

Cops say Skai and her BF both denied anything got physical -- despite the video -- and told officers they were happily engaged and expecting a baby.

Skai was still arrested for misdemeanor domestic battery, but she was released just a few hours later.