A staff member at Hudds Farm in Bradford-on-Avon said she was thrown from a horse when a Chinook helicopter flew overhead last Friday.
The yard's owner Bernice Marshallsay said she knew there was a military exercise but had no warning about the aircraft.
The MoD says it is doing all it can to minimise disturbance to the public.
Vicky Fielding, 20, suffered bruising when she was thrown from the horse.
"She panicked straight away and there was nothing I could do," she said.
"I'll be anxious if I hear another Chinook coming over."
Ms Marshallsay said: "We get no information at all and I've complained to the MoD on at least two other occasions and it still keeps happening.
"It's not good enough. Somebody's going to be seriously injured before something's done about this."
A spokesman from the Ministry of Defence (MoD) said it was raising the aircrews' awareness of horse riders.
It is doing all it can to minimise disturbance to the public while crews train for operational duty, the spokesman added.
The owner of a livery yard in Wiltshire has said low flying aircraft could cause someone to be seriously hurt.