Northwestern kept pace in the first half behind four 3-pointers as the Spartans shot 1 of 11 from 3-point range before halftime. MSU led 26-22 at the half.
The Spartans began to pull away with Harris hit a pair of free throws with 13:27 to go in the second half. A four-point lead grew in to a nine-point lead in less than two minutes and MSU rolled.
Alex Gauna tied a career-high with eight points.
Michigan State was without forward Adreian Payne, who missed his second straight game with a foot injury. Forward Branden Dawson did not start as he has been day to day with a virus.
Coach Tom Izzo moved into a tie for fourth place in Big Ten history with 214 conference wins.
The Spartans head into Illinois on Saturday night with a 16-1 record and 5-0 in conference. They also have won all four true road games by 14 points or more.