A deft back-heeled goal from defender Matthias Ginter and impressive finishing by Leon Goretzka and Kroos gave the Germans a comfortable 3-0 lead early in the second half in Moenchengladbach.
Germany captain Manuel Neuer produced a superb save to keep out Belarus striker Igor Stasevich's penalty before Kroos dribbled through the defence to claim his second goal seven minutes from time.
The result means victory in Frankfurt over Northern Ireland, who drew 0-0 at home to the Dutch on Saturday, will send Germany to next year's finals as Group C winners.
In the pre-match build-up, head coach Joachim Loew insisted his fledgling Germany team can not be considered among the favourites for the Euro 2020 title next July.
There seemed to be some truth to that as a poor first-half performance saw Germany struggle to break down the visitors' dogged defence despite 80 percent possession.
Long-term knee injuries in recent months to Bayern Munich defender Niklas Suele and Manchester City winger Leroy Sane have robbed Loew of two key components of his new-look side.
Bayern Munich winger Serge Gnabry and RB Leipzig striker Timo Werner led the Germany attack.
Just after Belarus had tested Neuer when Stasevich managed a rare shot on the German goal it was Gnabry who helped put the hosts ahead.
His well-timed cross gave Ginter the chance to conjure up an innovative finish as he flicked the ball past Belarus goalkeeper Aleksandr Gutor on 42 minutes.
Germany doubled their lead four minutes after the restart when Goretzka drilled a Kroos corner into the top corner.
Kroos claimed Germany's third goal when Ginter passed across to him and the Real Madrid star casually hit the back of the net from the edge of the area on 55 minutes.
Neuer proved he is still one of the world's best goalkeepers when he denied Stasevich from the spot 15 minutes from time following a foul by Robin Koch on Pavel Nekhaychik.
Kroos netted again as he beat two defenders and sent Gutor the wrong way as Germany moved five points clear of third-placed Northern Ireland with one game to play.