The officer suffered an injured arm after being thrown "like a rag doll" through the air in the crash next to the busy airfield on Sunday (July 20).
His motorbike was in collision with a BMW 1 Series car at the Meadow Gate roundabout at the exit onto Elles Road, on the boundary of Farnborough Airport, just after midday.
Sean Cooke, 46, from Deepcut, was next to the roundabout waiting for the flying display to start, and had set up some chairs for his wife and three children.
He said the officer’s marked bike, a BMW RT1200, hit the driver's side window, smashing it and leaving the motorist with cuts to his arm.
“I was pretty much first on the scene,” he said. “I took his helmet off and loosened his clothing around his neck area.
“He had limped over to the barrier where his bike had crashed into.
"He flew around seven or eight feet [in the air]. He looked like a rag doll. I was thinking the worst until the helmet came off.
“I wanted to do my bit. I have a scooter and if it happened to me I’d like to think someone would do the same.”
Mr Cooke said he understood the officer had been on his way to deal with some traffic congestion nearby at the time of the collision.
A Hampshire Constabulary spokesman said the motorcyclist was taken by ambulance to Frimley Park Hospital with an injury to his arm.
The driver of the BMW, a 28-year-old man from Yateley, was also taken to hospital.
Road closures were in place following the incident and the road was reopened just after 4pm.
Anyone who witnessed the collision is asked to contact Sergeant Adrienne Jerram from the Roads Policing Unit at Eastleigh police station by calling 101 and quoting Operation Cornwallis.