Arrieta (2-1) retired eight of the first nine batters. Acquired from the Baltimore Orioles on July 2, Arrieta has been inconsistent in his four starts since joining the rotation Aug. 16. But this time he worked around three walks with only two Phillies advancing beyond second base in his 6 2-3 innings.
Three Cubs relievers combined for 2 1-3 shutout innings. The victory gave the Cubs their first consecutive home wins since a stretch of three straight July 6-9.
The Cubs came through against Phillies starter Kyle Kendrick (10-12).
Ryan Sweeney, activated from the 60-day disabled list Sunday, delivered in his first at-bat since breaking a left rib when he crashed into the center field wall June 29 in Seattle. Sweeney's single to center scored Darwin Barney from second to give the Cubs a 1-0 lead in the first inning. Kendrick snared Donnie Murphy's liner to strand runners on the corners to end the inning.
Anthony Rizzo's double off the right field wall - just out of reach of a leaping Darin Ruf - scored Starlin Castro from first extending the Cubs' lead in the third.
Ruf got a run back in the fourth, hitting a solo homer off Arrieta to cut the Phillies' deficit to 2-1. The Phillies have lost 21 of their last 23 games when scoring three runs or less. Kendrick yielded five runs on eight hits in six innings.
The Cubs then broke the game open in the bottom half, starting the inning with three consecutive hits, including back-to-back doubles by Brian Bogusevic and Castillo. Castillo's double to left-center field drove in two runs. Arrieta hit a flyball to right and Castillo advanced to third.
Kendrick struggled with his command, missing in the strike zone and then hitting Castro with one out to put runners on the corners.
Ruf delivered an on-target throw home on Barney's sacrifice fly to right field, however, Phillies catcher Erik Kratz couldn't hold onto the ball when he went to tag Castillo, giving the Cubs another run and a 5-1 lead.
Pinch-hitter Luis Valbuena, who was activated from the 15-day DL before the game, drew a bases loaded walk in the eighth against Phillies reliever J.C. Ramirez and Castro added a sacrifice fly for two insurance runs and a 7-1 lead.