A watchdog report found there were 554 self-harm incidents at Brinsford young offenders institution between May and October last year.
A small number of individuals accounted for multiple incidents.
Publishing the assessment, Chief Inspector of Prisons Peter Clarke said: "In order to understand the dreadful increase in self-harm, it is impossible to ignore the potential impact of the regime at Brinsford, which was particularly poor for a population consisting mainly of young adults.
"For those who were supposedly in full-time employment, five and a half hours out of their cell each day was typical, and was simply not good enough, leaving very little time for access to showers or telephones."
Located near Wolverhampton, HMYOI Brinsford holds young male inmates aged 18-21.
At the time of the inspection in November the facility had a population of 473.
HM Inspectorate of Prisons concluded that "boredom and frustration" contributed to high levels of violence at the establishment.
However, the report noted that Brinsford had managed to avoid the "enormous increases" in violence that have hit much of the prison estate.
Mr Clarke said: "For the moment, Brinsford is a prison that is working hard to bring about some much-needed improvements, which we hope will prove to be more durable than in the past."
Michael Spurr, chief executive of HM Prison & Probation Service, said: "As the Chief Inspector acknowledges, the Governor has robust plans in place to improve performance and safety at Brinsford.
"Constructive activity has increased since the inspection and prisoners have more time out of cells. Systems to support the most vulnerable and to reduce self-harm have been strengthened.
"Staff and managers are determined to achieve the sustained improvements required and progress will be closely monitored over the coming months."
Copyright (c) Press Association Ltd. 2018, All Rights Reserved. Picture (c) David Jones / PA Wire.