Brazil superstar Neymar and World Cup winner Kylian Mbappe watched from the stands as PSG provided perfect preparation for the Champions League opener at Anfield, but the result somewhat flattered the hosts after a shaky first 45 minutes.
Les Verts created several gilt-edge opportunities to open the scoring, with Lois Diony spurning the best chance from six yards before Julian Draxler broke the deadlock after 22 minutes.
St Etienne continued to threaten until Edinson Cavani converted a 51st-minute penalty for his third goal in three games before Angel Di Maria and Moussa Diaby's first goal for the club wrapped up maximum points.
And Tuchel was impressed as he watched his side secure their fifth triumph in five league matches without his star duo.
The German head coach told the club's official website: "I'm very happy with this victory, our sixth in a row since the start of the season.
"It's important to several options when we play. Today we were without Neymar and Kylian and I'm happy to have used players that were here during the international break.
"It was a difficult first half and Saint-Etienne played very well. We lost too many easy balls and that gave our opponents solutions.
"In the second half, we attacked down the wings more and we found more space for Di Maria and Julian Draxler.
"It was a good rehearsal, good for the confidence ahead of Tuesday's match in Anfield against Liverpool. It will be a big test for us, but we are confident."
St Etienne suffered their first defeat of the season – having gone into the match with one win and three draws – but head coach Jean-Louis Gasset believed it was a missed opportunity to take points from the reigning Ligue 1 champions.
He told www.asse.fr: "Based on the first period, the score is very hard on us.
"If we do not hurt PSG when they are in doubt, it's very difficult.
"At 1-0, I wanted to stay in the same pattern and not concede another goal. Yet this goal happened too soon and it hurt us.
"From there, Paris gained confidence, which allowed them to show how huge the gap between the two teams is.
"It's been three games that we do not score. From the moment we we went behind, it was necessary to change the system, which left one-on-one situations with players like (Adrien) Rabiot, (Marco) Verratti, (Angel) Di Maria and (Edinson) Cavani. There we saw the difference of class."