A low-key start to the match saw both teams struggle to threaten, but it was Saint-Etienne that made the breakthrough in the 17th minute courtesy of a close-range finish from Cabella.
The goal was a mess from PSG's point of a view as the league leaders failed to clear a dangerous cross from Debuchy, and Cebella accepted the gift to send the home side into the lead.
Saint-Etienne had a golden chance to double their lead in the 31st minute when Presnel Kimpembe was adjudged to have felled Romain Hamouma inside the penalty box, but Cabella saw his spot kick saved by Alphonse Areola.
PSG were fortunate not to concede a second, but Emery's side went down to 10 men in the 41st minute when Kimpembe picked up his second yellow card for a rash challenge.
Cabella came close to doubling Saint-Etienne's lead early in the second period when the attacker worked some space in an advanced area before curling towards goal, but the ball just flew wide of the post.
Edinson Cavani came off the PSG bench in the 68th minute, but it was the home side that continued to look the most likely, with Oussama Tannane rattling the crossbar in the 74th minute following good work from Cabella.
Cavani had a wonderful chance to level the scores 15 minutes from time when Di Maria teased a low cross into the penalty box, but the Uruguay international somehow missed the post from close range.
Cavani passed up another opportunity in the 91st minute when he fired at Stephane Ruffier from close range, but PSG had their equaliser in spectacular fashion one minute later when a cross from the right hit Debuchy and ended up in the back of the net to silence the stadium.
SAINT-ETIENNE (4-2-3-1): Ruffier; Debuchy, Perrin, Subotic, Gabriel; M'Vila, Selnaes; Monnet-Paquet (Ntep de Madiba 57'), Cabella, Bamba (Pajot 83'); Hamouma (Tannane 68')
PARIS SAINT-GERMAIN (4-3-3): Areola; Meunier, Marquinhos, Kimpembe, Kurzawa; Lo Celso, Diarra (Silva 46'), Rabiot; Pastore (Cavani 68'), Mbappe, Di Maria