Raonic, from Thornhill, Ont., won the first set and then had to remain focused for half an hour as the roof of the Ariake Colosseum was shut due to drizzle as a typhoon moves towards Japan.
"I did what I had to do well today," said Raonic. "It was about taking advantage of my opportunities. Against Simon I had to lift my game."
Raonic has yet to win the Japan Open title, losing last year in the final to Argentina's Juan Martin del Potro and in 2012 to Japan's Kei Nishikori.
Later on Saturday, the fourth-seeded Nishikori beat Germany's Benjamin Becker in the second semifinal. That sets up a rematch of 2012's final with Raonic on Sunday.
"I'm just concentrating on each match," said Raonic. "The goal now is to try and win this title but it won't be easy.
"I need to come out and do everything well."
The win improves Raonic's chances of qualifying for the ATP World Tour Final, an elite eight-man season-ending tournament to be held next month in London, England.
The 23-year-old Raonic, who just missed the cut last year, currently holds the eighth spot in the standings. There are seven tournaments left on the ATP calendar over the next four weeks.
"I've been playing well this week and playing as well as I need to," said Raonic. "I'm not thinking about the year-end event in London, it's still too far away."
Raonic improved to a perfect 3-0 record over Simon, a former top-20 player now ranked 37th.
Raonic was impressive in the victory, starting the match with two aces in the opening game and breaking for 5-1 in the first set.
A game later, after 31 minutes on court, the Wimbledon semifinalist had the set in the bag only to find himself off court for the 30 minutes it took to close the roof and start the warm-up again.
Simon steadied in the second set, but was soon broken to give Raonic a 3-2 lead after a double fault.
Raonic earned the win with his 11th ace of the afternoon to secure three match points. Simon saved one before going down to defeat.
Raonic ended without facing a break point while breaking his opponent three times. He improved his 2014 record to 32-13 with a title in Washington and three high-profile semifinals at Rome, Wimbledon and Cincinnati.
Raonic will be playing in his 12th career ATP final as he bids for his seventh trophy. He's reached the quarter-finals or better at 11 of his 14 tournaments this season.
Note to readers: This is a corrected story. A previous version said Raonic had qualified for the ATP World Tour Final.