New signings Aliou Cisse and Kenny Cunningham were not involved at The Lawn but Robbie Savage was selected by Steve Bruce.
Blues took the lead after 10 minutes when Stern John followed up an effort from Geoff Horsfield to score.
A mistake by Forest Green's Gary Owers allowed the Blues to swarm forward and Bryan Hughes took full advantage to double the lead.
Stan Lazaridis then scored twice, the first in the 22nd minute after John's pace cut through the home defence and the second a tap-in five minutes before half-time.
There was still time for one more before half-time, new-boy Savage scoring from the spot after he had been tripped in the box.
The final goal again came from Hughes in the 80th minute after the pacy Lazaridis had led a breakaway.
Blues are yet to concede a goal in their two pre-season friendlies so far, and up to now have scored seven goals.
Squad: Bennett, Vaesen, Kenna, Purse, Tebily, Eaden, Grainger, Michael Johnson, Hyde, Devlin, John, Damien Johnson, Horsfield, Hughes, Lazaridis, Savage, Williams, Woodhouse.