The actor Johnny Depp will fire the cannon to spread the ashes of the author of Fear and Loathing in Las Vegas, who shot himself dead on February 20 at his home near Aspen.
Thompson was 67 and had been in failing health.
Depp, who appeared in the film version of Fear and Loathing, paid for the construction of the wooden cannon for the ceremony that Thompson had always said he wanted. He played Thompson in the movie and became a close friend of the writer.
Today is the six-month anniversary of his death.
"I think he had a brilliant imagination and this (funeral) was something of a fantasy," Troy Hooper, associate editor of the Aspen Daily News, said.
About 300-350 friends and relatives are expected to attend the private ceremony on the grounds of Thompson's home in Woody Creek, just outside of Aspen, the posh Colorado mountain resort.
Thompson's ashes will be scattered from the cannon on a tower topped by a double-thumbed red fist the symbol of Thompson's gonzo journalism just as the moon rises.
The Kentucky-born Thompson created gonzo journalism a form of story telling where the author is part of the story in the 1970s. But besides his creativity, he was also known for hard living.
"He'd be drinking all the time. In the morning he'd pour a glass of scotch, smoke a cigarette, eat devilled-eggs and go from there," Mr Hooper said. One line covered all vices. "I hate to advocate drugs, alcohol, violence or insanity to anyone, but they're always worked for me."