The victim was struck by the 6ft by 7ft site perimeter fencing panel when it fell in high winds.
Main contractor on the job is Thomas Vale who are converting the historic Piano Building into a new academy for Kidderminster College and Birmingham Metropolitan College.
The woman was waiting at bus stop at the time of the accident and had to be freed from under the hoarding by members of the public.
A West Midlands Ambulance Service spokesman said: “Ambulance crews arrived to find a female had been struck by a metal fence panel.
“A woman in her fifties was treated at the scene for a serious head injury.
“The patient was immobilised with the use of a neck collar and placed on a spinal board before being taken to the Queen Elizabeth Hospital in Birmingham for further assessment and treatment.
“The hospital was alerted to the serious condition of the patient.”
A Thomas Vale spokeswoman said: “Due to adverse weather conditions, we understand that a section of fencing collapsed which caused some injuries to a member of the public on the perimeter of the site.
“The emergency services, alongside the Health & Safety Executive were notified immediately and we will be working with all parties to determine the root-cause.
“We wish the lady in question, a speedy recovery.”