Emergency services were called just before 1pm today to the incident.
The road remains closed while emergency services work at the scene.
A Cambridgeshire police spokesperson said; “We were called at just before 1pm today to the A1 northbound, just past Kate’s Cabin Café, with reports of a collision between a lorry and a pedestrian.
“The pedestrian, a man, has suffered serious injuries and officers and paramedics remain at the scene.
“The road is currently closed northbound at junction 17, and is likely to remain closed for some time. Motorists are urged to avoid the area and seek alternate routes.
“Anyone with information is urged to contact DCI Gary Webb of the BCH Road Policing Unit via web chat: https://bit.ly/3vsiYDw or online forms at www.cambs.police.uk/report quoting incident 178 of 29 October. Anyone without internet access should call 101. Always dial 999 in an emergency.”