"Why did he lie to Congress (jail)," Mr Trump said, adding: "How come he gave up classified information (jail)."
He also criticised Mr Comey's forthcoming memoir, saying the "badly reviewed book" raises "big questions".
His comments come ahead of an ABC News interview with Mr Comey to be aired on Sunday as part of his book tour.
The book - A Higher Loyalty: Truth, Lies and Leadership - reportedly paints an unflattering portrait of Mr Trump, who fired Mr Comey last year.
According to excerpts of the book obtained by US media, Mr Comey writes that interactions with Mr Trump gave him "flashbacks to my earlier career as a prosecutor against the Mob. The silent circle of assent. The boss in complete control. The loyalty oaths. The us-versus-them worldview".