Eavis had to draw up a special document featuring glowing quotes from past headliners to persuade The Boss to agree to play.
"It's been our mission for quite a long time to get Bruce," she told BBC 6music. "We put together a document for him and spoke to his people a lot and they are really up for it. He's never done a festival before so we didn't expect him to know much about it."
"We put together some information and said 'This is what happens, this is all the money that goes to charity'. Pretty quickly he said: 'Yes'. It's amazing."
The Boss will headline the festival's Saturday night, on 27 June.