The Montana Mining Division said the worker was standing in a drip pan in a silo when he fell about 16ft onto a safety guard.
It is believed the welding holding the drip pan broke.
The worker was responsive while being taken to hospital Friday, according to local news station KULR8.
Bull Mountain is Montana’s only underground mine and employs about 325 workers.
It has previously reported three accidents to the Mine Health and Safety Administration this year.