The busy road was closed in both directions for several hours on Sunday night (September 29) while emergency services dealt with the incident.
A woman was airlifted to hospital with chest and abdominal injuries.
While a man, who suffered pelvic injuries, and two children, who were also injured in the crash, were taken to the Royal Victoria Infirmary, in Newcastle, by road.
Emergency services were called to the scene of the crash – which involved multiple vehicles – at around 6.15pm on Sunday.
An air ambulance, two road ambulances, two clinical care managers and a HART vehicle were sent to the scene. Northumbria Police and Northumberland Fire and Rescue Service also attended.
A spokesperson for the Ambulance Service said: “There have been a number of casualties, with some trapped.
“A woman has been air lifted to the RVI with chest and abdominal injuries.
“A man with pelvic injuries along with two children who are also injured have been taken by road to the RVI.”
The road was closed for around eight hours in total as a clean up operation and investigations by police took place.
A diversion was put in place and motorists who were travelling in the area were being diverted through Belford’s local roads.
The A1 has now been fully reopened.
A Northumbria Police spokesman said: “The A1 north and south at Belford by-pass has now been re-opened following a collision. Thanks for your patience.”