Beltre joined the Rangers after spending the 2010 season with the Red Sox.
The blast was Beltre's only hit, but it was the biggest of the Rangers' 15-hit attack. all nine players in the lineup registered at least one hit. Nine Ranger hits went for extra bases, including four home runs.
The Rangers have taken the first two games of the series from Boston and will look to complete the season-opening sweep on Sunday.
During last year's playoffs and World Series, Cain had thrown 21 1/3 innings without surrendering an earned run. On Saturday, he allowed five hits, issued no walks and fanned three.
Freddy Sanchez provided a nice dose of offense, going 3-for-4, with a single, double and home run. Sanchez drove in three runs and scored twice as the Giants beat up Dodger starter Ted Lilly and three relievers.