A a firework shot into the crowd at the display in Brunswick Park in Wednesbury.
Three people were injured including a 23-year-old man who suffered an ear injury. He was taken to Walsall Manor Hospital for checks.
A seven-year-old girl and a 47-year-old man were treated by St John Ambulance staff for minor leg injuries but did not require hospital treatment.
The display on Wednesday night had attracted around 12,000 people – twice the number who attended the previous year.
Sandwell Councillor Maria Crompton, whose department oversaw the event, said: "We are thoroughly investigating exactly what happened at Brunswick Park."
She added displays in West Bromwich and Smethwick would go ahead tonight (FRI) as planned.
Numerous calls were made on the public address system for people to move away from the fireworks before the display started. The event was delayed for 20 minutes as the crowd was forced back.
Elaine Costigan, ward councillor for Wednesbury North, blamed a rogue firework for the incident and claimed safety had been a priority for organisers.
She said: "If you get a rouge firework, what can you do. There were a lot of people there but even if it had been 1,000 in the crowd you can still get a rouge firework.
"People are still saying what a fantastic event and exhibition it was. We have to look forward now."
Emma Flemming, from Walsall, was watching the show with her four-year-old son, and was stood close to where the firework landed.
She said: "I saw the firework and didn't know whether it had already gone off or was about to go off.
"I grabbed my son and threw him out of the way to put myself between him and it. I could feel the heat hit my face when it went off.
"I thought it had gone off in my son's face."
Chris Ringham, from Walsall, was at the event with his fiancée Stevie and two year-old son Joel.
He said: "I saw it go into the crowd. As soon as this firework went off everything stopped.
"The crowd started dispersing quite quickly. You could see there were people who were distressed."
"They were trying to move people back. But there were still people in the zone they didn't want people to go."