It was Saint-Etienne that had the first half-chance chance of the match in the ninth minute, but Saidy Janko, who had found space in a central position, blasted his effort over the crossbar.
PSG were warming to their task as the first period developed, however, and the hosts took the lead from the penalty spot in the 20th minute courtesy of a well-taken strike from Edinson Cavani, who had been felled inside the box by Janko.
Saint-Etienne reacted positively to falling behind, and Jonathan Bamba forced Alphonse Areola into a smart save in the 25th minute, before Janko sent a powerful effort over the PSG goal in the 31st minute as the visitors continued to threaten.
PSG doubled their lead early in the second period, however, with Thiago Motta finishing into the bottom corner past Saint-Etienne goalkeeper Stephane Ruffier after Marquinhos had reacted first to a free kick from Neymar.
Angel di Maria was next to come close for PSG - hitting the post with a 35-yard free kick 25 minutes from time, before Cavani blasted over the crossbar 10 minutes later after being presented with a golden opportunity inside the Saint-Etienne box.
Javier Pastore was next to pass up a decent chance for the hosts after missing the target in the 81st minute, but Emery's side did make it 3-0 in the final minute of normal time when Cavani back-heeled a cross from Julian Draxler into the back of the net.