Landon Donovan led the attack on Sunday by scoring one goal and helping set up three others for the Americans, who reached the semi-finals of the biennial North American regional tournament for the seventh-straight time.
"Landon again today proved how valuable he is and how he can make a difference," said USA coach Jurgen Klinsmann.
Donovan set up the first goal in the 21st minute with a cross to Clarence Goodson, who one-timed a shot home from six yards out at M&T Bank Stadium.
US goalkeeper Nick Rimando made a brilliant double stop a few minutes later to preserve the Americans' lead.
Joe Corona made it 2-0 by eluding a defender and slotting his shot inside the left post.
Rodolfo Zelaya got El Salvador back in it with a goal before half-time when he drew a foul on DaMarcus Beasley inside the penalty area. Zelaya successfully converted the spot kick.
USA's Eddie Johnson came off the bench in the 60th minute to score just seconds after stepping onto the pitch by heading home Donovan's corner kick.
Johnson also had a hand in the American's fourth goal. His flicked header in the 78th minute sprung Donovan, who got possession of the ball and stepped nicely around the goalkeeper before rolling it into the empty net for a three-goal lead.
Mikkel Diskerud capped the scoring six minutes from time as he grabbed a Donovan cross and slotted it in to make it 5-1.