Speaking in a Facebook video he says, "The A350 is not an aircraft we will buy. Too expensive. Fares would go up."
AirAsia X is understood to have been eyeing an order for additional A350s or Boeing 787s to complement its fleet of A330-300s, and 66 on-order A330-900s.
Its 10 A350-900s on order are scheduled to start delivering in 2019, Flight Fleets Analyzer shows.
In the same video, however, Fernandes also appeared to throw cold water on a return to flying to London, saying that there were "no plans" to resume services to the UK capital.
His comments appear to contradict comments from carrier’s head of network and regulatory Venggatarao Niadu, who recently indicated that the carrier would look to expand its network to Europe and the United States “in about 2019”.
AirAsia X previously flew from Kuala Lumpur to London and Paris using A340s, but those routes were dropped in 2012.
Airbus indicates that an A350-900 costs around $317 million at list prices.