The former Bayern captain, who lifted the UEFA Champions League in the club's 2012/13 treble-winning season, has urged caution despite their impressive progress so far.
Bayern reached the semi-finals earlier this month after a thrilling 4-3 second-leg win over Real Madrid, securing a 6-4 aggregate victory.
Lahm praised Bayern's attacking output in that tie, but warned that defensive vulnerabilities could undermine them against PSG.
He highlighted concerns over Bayern's reliance on one-versus-one defending, suggesting it leaves them exposed at the highest level.
Instead, he advocated for a more structured zonal defensive system, arguing it would provide greater stability in high-pressure matches.
"They (PSG) have incredible individual quality and are the defending champions. Above all, [Bayern] must be very, very solid defensively," Lahm said.
"Bayern are scoring goals both in the Bundesliga and the Champions League, but there is a danger against PSG defensively. They (PSG) have incredible attacking power, so that will certainly be a decisive factor. Defensively, they need to stand firm and defend well."
He added: "They have unbelievable quality in attack, but they often defend one against one, which can be dangerous against top teams like PSG. If you don't have the direct support of your teammates, I see a certain danger in the Champions League matches against PSG."
Paris Saint-Germain manager Luis Enrique has insisted no side in Europe is better than his team as the Champions League holders prepare for a huge semi-final first leg against FC Bayern Munich in Paris on Tuesday night.
Harry Kane has been praised as a player reaching new heights, with Vincent Kompany insisting the Bayern Munich forward was still improving ahead of their Champions League semi-final against Paris Saint-Germain in Paris tonight.
Bayern Munich are preparing to open contract talks with Harry Kane at the end of the season, as the club looks to secure the long-term future of their prolific striker.
Bundesliga champions Bayern Munich showed why they are the best team in Germany by coming from three goals down to beat Mainz 4-3 at the MEWA Arena on Saturday.
