John Gillion Watson, 56, of Vicarage Court, Sleaford, pleaded guilty to a charge of arson being reckless to as to whether life was endangered on March 29 last year.
Watson had previously pleaded not guilty to a more serious charge of arson with intent to endanger life on the same date.
The unit was evacuated with no injuries to patients or staff but ambulances were diverted to other hospitals in Lincolnshire, as well as further afield.
Damage of £180,000 was caused to Lincoln County Hospital. It also caused disruption to the neighbouring x-ray and scanning department until mobile scanning equipment could be brought in.
Luke Chisnell, prosecuting, said Watson's plea was acceptable and the Crown Prosecution Service would not seek a trial on the charge of intending to endanger life.
Michael Cranmer-Brown, defending Watson, said the defence were seeking to obtain a psychiatric report on Mr Watson.
Judge Catarina Sjolin Knight adjourned sentence until March 20 and remanded Watson back into custody.
The Judge indicated: "I am concerned about dangerousness for obvious reasons."
She told Watson: "You have pleaded guilty to what is a very serious offence. Who ever sentences you needs to know a lot more about you."