Antoine Griezmann scored his first goal for the club since his €120 million ($198.9 million) move from Atletico Madrid, in between strikes from Luis Suarez.
Ousmane Dembele added a fourth against Carlo Ancelotti's side, which lost the first friendly meeting in Miami 2-1 on Thursday (AEST) and was simply no match for Barca here once it fell behind.
It was an encouraging display from Ernesto Valverde's side, without the injured Lionel Messi, ahead of its LaLiga opener at Athletic Bilbao on Saturday (AEST).
Griezmann dragged a clear chance wide and Dembele also fluffed his lines as Barca failed to capitalise on first-half openings.
Suarez was denied by Alex Meret, but Fabian Ruiz went within inches of giving Napoli a lead at the break.
Griezmann spurned another chance three minutes after the break, firing straight at Meret, but Suarez was on hand this time to side-foot home the rebound.
Barca turned on the style for its second eight minutes later. A passing move starting from the back four ended with Carles Alena sliding a superb throughball to Jordi Alba, perhaps fortunate not to be flagged for offside, who squared for Griezmann to bury an easy finish.
It got worse for Napoli just two minutes later, as Dembele picked up Suarez's pass from a loose ball and teed up the Uruguay star for a stunning curling strike that flew past Meret from the edge of the box.
Dembele marked a strong second-half showing by driving into the penalty area and drilling a right-foot strike into the bottom-left corner, ensuring Barca will head into the new season in positive fashion.