Washington State (2-1) also got a long interception return for a touchdown from Damante Horton for the second consecutive game.
Southern Utah (2-1), an FCS program, was scoreless in the second half and gained just 219 yards in the game.
Halliday completed 32 of 41 passes and was intercepted once. Washington State gained 464 yards of total offense in the game. Dom Williams caught three passes for 101 yards and two touchdowns.
Aaron Cantu completed 22 of 30 passes for Southern Utah, but was intercepted three times.
Washington State was coming off a 10-7 win at Southern California that knocked the Trojans from the Top 25.
Southern Utah beat an FBS team, South Alabama, in its season opener, but had less success against Washington State of the Pacific-12.