The hosts, who are currently second from bottom in the top division, upset the form book with goals from Marek Bakula, Dan Whitelaw, Charlie Hassell and Tom Hopper.
“I have been involved with Ringwood for many years and cannot recall another youth side reaching the final,” said manager Paul Hopper. “It will be a great day for the boys and the club.”
Ringwood’s joint coach Chris Till added: “Prior to this game we have had an unlucky season and our league position does not reflect what we are capable of.”
It looked as if Ringwood’s bad luck had struck again after only eight minutes when Joe Till limped off after a heavy tackle.
But it was the home side who broke the deadlock when Bakula used his pace to latch on to a loose ball and plant the ball in the back of the net.
Bransgore, however, hit back when Liam Lavis converted a looping cross at the back post.
Ringwood rallied again to regain the lead when Whitelaw headed in George Best’s free-kick.
Bransgore stepped up a gear after the interval but were caught on the break when Hassall netted following good work from substitute Asa Drodge.
Oran Dougan brought Bransgore back into the game before Hopper got the final touch to score Ringwood’s fourth.
Bransgore boss Richard said: “As always, it was a hard fought game between two very good sides, although the lack of games for both sides affected the quality of the match.”
Ringwood will meet Green-fields, who were beaten finalists last year, in the Paul Allan Trophy final on April 6.
Ringwood: Jordan Manley, Tom Shorey, Laurie Smith, George Best, Marcus Cole (captain), Tom Hopper, Harrison Clarke, Joe Till, Dan Whitelaw, Charlie Hassall, Marek Bakula, Asa Drodge, Brandon Kirby.
Bransgore: Paul Sutton, Tom Cowen, Eddie Myers, Nathan Mcaullife, Connor Lopez, Josh Moody, Oran Dougan, Dan Smith, Tommy Hall, Liam Lavis, Ryan Pimble (captain), Dan Tennant, Scott Dyson, Kayde Dilnott, Jamie Field.