The 17 partygoers were travelling on a trailer pulled by a tractor driven by another person, said Aargau police.
At around 7pm, as the vehicle went down a steep slope, the heavy trailer pushed the tractor forward and the driver, despite braking, lost control.
Both the tractor and the trailer tipped over, injuring all 18.
Three of them received serious head, back and pelvic injuries, said police.
Two rescue helicopters, police and fire fighters attended the scene. All the injured were transferred to eight hospitals in the region.
The area around the Hochwachtstrasse was closed for several hours.
The public prosecutor has opened an investigation into the causes of the accident.