The actor told the Boston Herald that the sci-fi drama "picks up right where it left off".
Barrowman said: "We're still the same edgy Torchwood [and we] still have the edgy characters and storylines that people seem to like. It wasn't like a shock to the system [for us, and] our really core audience are not going to feel that [either]."
Barrowman, who plays Captain Jack Harkness, also claimed that he is not worried how new viewers will react to Torchwood.
"I don't worry about that," he said. "I know in my heart we've made a good show."
He added: "You know what, not everybody is going to like it, but I hope we get a bigger audience than what we've had before."
Torchwood: Miracle Day begins on July 8 on Starz in the US and on July 14 on BBC One in the UK.
> Torchwood: Miracle Day: Ten teasers
Watch John Barrowman discuss Torchwood: Miracle Day with Digital Spy below: