Eight people had to be treated by ambulance staff after a firework misfired across the front of the audience.
They were given eye wash treatments as a result of falling debris at the Staffordshire park on Sunday night.
Medical staff from St Johns Ambulance Service helped the wounded and one person was advised to go to hospital as a precaution.
Alton Towers said an inquiry found a manufacturing fault was to blame.
A spokeswoman said: "The Alton Towers' displays are professionally organised by a specialist company who, together with park management, immediately noticed the discrepancy and pulled the remainder of the fireworks from the show as a precaution.
"Obviously the health and safety of both park employees and guests is our primary concern and a full investigation was conducted last night, which concluded that there was a manufacturing defect with one of the fireworks which misfired, causing another one to fire at an incorrect angle."
It said it was the first incident in the park's 15 year history of putting on displays.
Source: BBC News