PSG are seeking a third successive treble having achieved the feat twice under Laurent Blanc and, though they face a stern challenge from Monaco and Nice to retain their Ligue 1 title, Unai Emery's men took another step towards a successful defence of this competition in Brittany.
The big-money signing of the January transfer window for PSG, Draxler has quickly justified the club's outlay and took his tally to four goals in five games with this double.
Draxler's exquisite lob broke the deadlock midway through the first half, and a straightforward finish from Lucas Moura made it 2-0 before the interval.
The visitors had chances to add gloss to the scoreline after the restart as Rennes never looked like mounting a comeback.
Draxler's curling effort put the game beyond doubt in the 68th minute and Hatem Ben Arfa rounded off the scoring, with PSG's attentions returning to the league on Saturday as they visit Dijon looking to cut the three-point gap to leaders Monaco and Nice.
Rennes were the first to threaten an opener when Giovanni Sio curled a left-footed free-kick narrowly over the bar.
But a largely dour first half was lit up in the 27th minute as Draxler collected a superb long ball from Thiago Motta, controlling the ball with the back of his heel and then lofting it over Benoit Costil and in off the post.
Lucas flashed an effort across the face of goal and wide of the left-hand post but he did double PSG's lead seven minutes before half-time.
Ben Arfa - given a rare start by Emery - produced an excellent low cross and Lucas arrived at the far post to tap in a simple finish.
Ben Arfa had the chance to turn from provider to goalscorer, but could not make proper contact with a delivery from the left as PSG's dominance continued.
Christopher Nkunku spurned a gilt-edged chance to make it 3-0 as he somehow fired wide from close range, with Rennes reduced to long-range efforts at the other end in their bid to get back into it.
Sio and Sanjin Prcic both tried their luck from distance, but the game was made safe by Draxler's assured finish.
The Germany international bent into the bottom-right corner from just outside the area and Julian Draxler PSG 01022017 added late gloss to the scoreline, slamming home a fourth after a quickly-taken free-kick.
Having progressed through this tie without having to use Edinson Cavani or Thiago Silva, PSG will have their big guns fresh for the next game in a compelling title race.