Despite facing the only side from the Premier League left in the competition, Mowbray is not fazed by the prospect of taking Pompey on.
"I don't think we've got anything to fear," he admitted.
"You can debate all day about the qualities of the Championship compared to the quality of the Premier League but the fact that none of the big four are left suggests that the underdog can win on any given day."
Albion last got to the final in 1968
It is Albion's second visit to Wembley since the new stadium opened and they are hoping for a better result than the last one they had there - the defeat to Derby in the Championship Play-Off final last May.
But Mowbray thinks Saturday's occasion will have a different feel.
"The Play-Off final is the last one of a three-day weekend and the pitch is sometimes beaten-up a little when you get to play on it," he added.
"But for the cup semi-final the pitch will be in pristine condition."
Albion v Portsmouth in the FA Cup semi-final is live on BBC WM 95.6FM from 1100BST on Saturday.
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