The invention discloses a method of implementing a fax in a next generation network, comprising steps of: either of two media gateways notifying a media gateway controller upon detection of a Fax Start signal when they communicate with each other; the media gateway controller sending in parallel a Modify command to the media gateways at the two ends of the communication based upon a negotiated fax codec mode, so as to modify the codec mode; the media gateways at the two ends modifying the codec mode based upon the Modify command, returning a Reply message to the media gateway controller, and starting a fax. The invention can reduce time for a fax switching and improve success ratio of a fax negotiation.