The invention relates to a service signal transmission method based on network protocol, to build and transmit voice, fax and data service on the IP successfully, and save bandwidth. Wherein, before building call, the caller and answerer consult the service type and the coding / decoding method; then normally building communication, and in the communication MGW detects the service instructions all the time, to report them to the MGC to judge the service type, and switch to relative decoding / coding method; based on fax or Modem service, and via character signal, judging if it is high-speed fax or Modem service, and using G.711Red code that supporting voice, fax and data; when it is high-speed fax or Modem service, switching into T.38 decoding / coding method; and judging the service type via the character signals as internal V21 and CM.