Michael Steven Sandford, 19, who is believed to be British, is expected to appear in a Nevada court shortly.
According to the court papers, he went to a Trump rally on Saturday at the Treasure Island Casino.
He reportedly approached a Las Vegas police officer to say he wanted an autograph from the candidate.
He has a UK driving license and told police he had been in the US for one and a half years, the court papers say.
The detainee reportedly told police he had been planning to try and shoot Mr Trump for about a year but had decided to act now because he finally felt confident enough to do so.
He said he had never fired a gun before but went to a range in Las Vegas on 17 June to learn how to shoot, according to the court papers.
At the rally he allegedly tried to grab the officer’s weapon because it was in an unlocked position and therefore, he said, the easiest way to get a gun to shoot Mr Trump.
Court Documents say Mr Sandford acknowledged he knew he would only be able to fire one or two rounds, and expected to be killed during an attempt on Mr Trump’s life.
He told police if he had not tried to kill Mr Trump at this rally he would have tried again at a rally in Phoenix, for which he had already booked tickets, the papers say.