It happened while the officer was making a traffic stop on the shoulder near Cole Mill Road.
Police said a tractor trailer rear-ended the unmarked police cruiser, which moments earlier had pulled over another vehicle.
Eyewitnesses reported that the tractor trailer swerved before striking the cruiser, spinning it out of its path. The impact trapped the female officer inside her vehicle.
Police said paramedics rushed the officer, who they haven't identified, to Duke Hospital where she was last listed in critical but stable condition.
The driver of the car which was pulled over by the officer was taken to Duke Hospital and later released.
The truck's driver remains at Duke Hospital. ABC11 has learned that he will be charged with driving while impaired, stemming from drug use.
Traffic on southbound I-85 was blocked for several hours.