The carrier initially launched the three-times-a-day service until 21 October while the Severn Tunnel was closed as part of the rail electrification programme.
But it said there had been a "very promising" uptake, with 95% of passengers surveyed saying they would continue using it.
The service has now been extended.
Tickets are now available for the coming weeks, with Cardiff Airport chief executive Roger Lewis saying "the immediate future" is secure.
He wants to see continued use to make it a long-term route at the airport.
Flybe's chief revenue officer Vincent Hodder said: "This route has quite simply taken off, quickly becoming an early success.
"We have been heartened by the response it has received."
The flight time is about an hour, but the record journey to date is 35 minutes.
Flights between Cardiff and London City airports will continue while demand exists, the airline Flybe has said.