A dominant first-half performance was brought to life after 18 minutes, when Isco nonchalantly curled a long-range effort into the top corner for his first ever international goal.
Sergio Busquets added a second within a minute courtesy of a powerful drive, while only the crossbar denied Paco Alcacer from getting in on the act before the interval.
The win was rounded off early in the second half when Pedro smashed home after some terrific work from Juanfran.
The result leaves Spain level on nine points with Ukraine in Group C, which is topped by Slovakia on 12, while Belarus remain without a win.
Vicente del Bosque, who made six changes from the win at Luxembourg, had described this contest as a potential ''litmus paper'' for his side.
And the 63-year old should have been celebrating an opening goal within the opening seven minutes.
A wonderful ball from Santi Cazorla was enough to slice open the Belarus defence and allow Pedro a clear one-on-one opportunity, only for the Barcelona forward to curl wide.
Cazorla was at the centre of proceedings again just before the 15-minute mark, but failed to really test Belarusian goalkeeper Yuri Zhevnov with a weak volley.
A Spain breakthrough seemed inevitable and it came four minutes later in quite sensational circumstances. A poor defensive clearance found Isco and the Real Madrid man picked out the top-left corner with a stunning 25-yard strike.
That opener visibly gave the home side more impetus and the lead was doubled just under a minute later.
A cross from the lively Juanfran could only be cleared as far as Busquets on the edge of the box and he made no mistake with a powerful low drive.
It was no less than the home side deserved in a dominant first-half display, with Sergey Balanovic's wayward 36th-minute strike representing the visitors' only threat.
There was little sign of Spain easing the pressure after the break, with Alcacer heading Pedro's cross onto the bar six minutes into the second half.
However, the home faithful did not have to wait much longer for a third goal, as a terrific move down the right involving Juanfran and Cazorla allowed Pedro to burst into the box and curl a vicious strike past the goalkeeper in the 55th minute.
Napoli's Jose Callejon came off the bench to make his full international debut after 69 minutes and the home side continued to control proceedings, Perdo striking the post late on.
Belarus applied some late pressure, but Del Bosque's charges were comfortable in extending their unbeaten home run in qualifiers to 29 matches - a run that stretches back to 2003.