Goalkeeper James Keating was the star of the shoot-out saving two kicks as Trinity won following a 2-2 draw after 90 minutes and 3-2 after extra time.
Bels played 70 of the120 minutes with ten men after striker Marc McGrath was sent off for serious foul play after a late challenge on Lee Jones.
Trinity manager Joe Morley said: ‘It was a great game played between two committed teams and full credit to Bels for going all the way after going down to ten players.
‘We had a great start with Barry Beatson nodding us ahead from his brother John’s cross only to switch off after half time a bit for them to equalise immediately after the resumption.
‘Both teams had some good spells and I was gutted when they scored their second goal deep into stoppage time to force extra time.
‘Again we were slow starters at the start of the first extra period and Bels went ahead.
In the second period Barry scored another outstanding goal – he had his back to goal when he attempted a bicycle kick and the ball flew into a top corner.
‘Penalty shoot-outs are a lottery and a goalkeeper usually comes out on top.
Fortunately for us it was James Keating on Saturday and we now have a final to look forward too’
Trinity’s second goal was scored by newcomer Ashley West who drilled a low shot into a bottom corner.
Trinity scored their penalties through Jimmy Reilly, James Hayward and Darren Swanson while Ray Grant hit the crossbar.
Trinity play holders Rozel Rovers or St Paul’s in the final at Springfield on Saturday 5 April.
Meanwhile, striker Dave Ibbotson hit a hat-trick to lift Jersey Wanderers out of the bottom two in Division I for the first time in several months.
Wanderers won 4-0 at the Hockey Club against Grouville leaving the parish side slipping into the second relegation place and now a point behind Wanderers.
Wanderers secretary and acting joint-team manager Peter Lucas said: ‘Dave ‘Gravey’ Ibbotson scored a super hat-trick and it was a vital win for us.
We’ve been playing well of late and we carried on from where we left off following our recent draw against Jersey Scottish.
First Tower suffered their tenth straight league defeat, 2-0 at La Hague Manor by the Portuguese recorded their goals through Carlos Carolo, together with an own goal.
Results: Coca-Cola Combination, Le Riche Cup, 1st rd: St Paul’s 6, St John 0.
Div I: First Tower 0, Portuguese Club 2; Jersey Wanderers 4, Grouville 0; Rozel Rovers 1, Jersey Scottish 3.
Div II: Beeches Old Boys 2, Sporting Academics 2; Jersey Nomads 2, St Ouen 2; St Clement 1, St Martin/SCF 1; St Lawrence 2, St Brelade 1.
Div I Res: Grouville 3, Jersey Wanderers 1; Jersey Scottish 3, St Brelade 5; St Paul’s 6, Rozel Rovers 2.
Div II Res: Beeches OB 2, St John 4; Sporting Academics 5, Jersey Nomads 1.
Div C: Grouville 1, St Brelade 2; St Martin/SCF 1, Rozel Rovers 3.
Ladies’ Div: Jersey Wanderers 1, First Tower 3; St John 4, Grouville 0; St Paul’s 2, St Brelade 0.
Jersey Telecom Junior League, U18 Div I: First Tower 3, St Brelade 1; St Paul’s 1, Jersey Wanderers 3.
Div II: Grouville 1, St Peter 5; Rozel Rovers 0, St Clement 3.
U16 Div I: St Paul’s scratched to Jersey Wanderers; Trinity 1, Grouville 1.
Div II: St Ouen 1, Rozel Rovers 1; St Peter 3, St John 0.
U14 Div I: First Tower 10, Rozel Rovers 1; Jersey Wanderers A 5, St John 2; St Paul’s A 2, St Clement A 3.
Div II: Jersey Wanderers B 0, St Paul’s B 10; St Brelade 1, St Clement B 2; St Peter A 6, St Ouen 2.
U12 Red Group: Rozel Rovers 3, Jersey Wanderers A 6; St John 2, St Clement A 3; St Paul’s A 3, St Peter A 2.
White Group: First Tower 1, Grouville 3; St Brelade 4, Jersey Wanderers B 2.
Blue Group: Grouville B 4, Beeches Old Boys 3; St Ouen 2, St Clement B 4.
