Arnold hit 7 of 11 shots and had a key block for the Broncos (2-0), who scored the first nine points of the second half and trailed just twice in the final 20 minutes. They made 11 of 15 free throws in the second half.
Anderson, the Broncos' leading scorer, had 14 points in the decisive second half after playing only six minutes in the first half due to foul trouble.
Darian Norris scored 16 points to lead San Diego (1-2), which led 32-27 at halftime.
Arnold made a big play when he blocked Matt Dorr's 3-point attempt with 10 seconds left and the Broncos leading 63-60. Dorr finished with 13 points.
Boise State held a 44-25 rebounding edge.