Spain were not at their fluent best during the first half and struggled to create many clear chances.
Indeed, Belarus may feel unlucky that they didn't get a penalty when Gerard Pique appeared to handle the ball inside the box just before the interval but the referee didn't award the spot kick.
Vicente del Bosque didn't wait to try to change the game, as he brought on Andres Iniesta at the start of the second half as he looked to earn a vital win in La Roja's quest to earn a place in Brazil next summer.
Spain kept looking for a breakthrough against a resilient Belarus defence but finally got their goal when Xavi Hernandez latched on to a loose ball before thumping it into the bottom corner of the net.
Alvaro Negredo, on as a substitute for debutant Michu early in the first half, made sure of the win with a goal 12 minutes from time thanks to a neat move started by a Pedro back heel and finished by a powerful header.
The Belarusians overall doggedness for much of the game was rewarded with a late goal from Sergei Kornilenko, which gave the hosts only brief food for thought.
The result means that La Roja only need to draw with Georgia on Tuesday to book their spot in the finals.