Plymouth started well, Paul Wotton forcing a fine save from Maik Taylor, before the Blues bossed proceedings.
Nicklas Bendtner scored the first when he took a Sebastian Larsson pass and rounded keeper Luke McCormick, who kept the score down with some fine saves.
Matthew Upson bundled in the second from a corner and Bendtner and Larsson laid on the third for Gary McSheffrey as the hosts took control of the match.
Plymouth manager Ian Holloway:
"We had a good start, could have scored early on, but then conceded the cheapest goal I've ever seen.
"We had some players out and struggled for a bit of balance but the lads who have come in will learn a lot today.
"Steve Bruce looked unhappy after 15 minutes but by half-time he had a big smile - his boys were doing him proud."
Birmingham: Maik Taylor, Kelly, Jaidi, Upson, Sadler, Larsson, Nafti, Clemence (Danns 82), McSheffrey, Bendtner (Campbell 77), Jerome.
Subs Not Used: Doyle, Muamba, Martin Taylor.
Goals: Bendtner 21, Upson 30, McSheffrey 41.
Plymouth: McCormick, Sawyer (Nalis 61), Seip, Doumbe, Capaldi, Norris, Djordjic (Aljofree 46), Wotton, Buzsaky (Samba 46), Summerfield, Hayles.
Subs Not Used: Larrieu, Gosling.
Ref: R Beeby (Northamptonshire).