Batting first after winning the toss, Afghanistan scored 194 for nine in 50 overs with Mohibullah Paak top scoring with a fine 44 which included five fours and a six.
For Sri Lanka, Lahiru Madushanka continued his fine form with the ball and finished with three for 34 while Amila Aponso took two for 19.
In its chase, Sri Lanka cruised to victory in 39 overs with player of the match Niroshan Dickwella hitting an elegant 76 and Pabasara Waduge contributing 33.
The two batsmen added 44 runs for the second wicket after Shehan Fernando (19) had departed at the score of 42. For the third wicket with Angelo Jayasinghe (23), Dickwella added 56 runs.
This was Sri Lanka's first Plate Championship title while for Afghanistan, it was an excellent achievement after it had finished 16th and last in its debut appearance two years ago in New Zealand.
Afghanistan captain Javed Ahmadi finished as his side's most successful batsman with 285 runs at 47.50 while Sri Lanka's leading run-getter was Sandun Weerakkody with 189 runs.
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On the bowlers' table, Sri Lanka's Madushanka finished with 15 wickets while Sayed Shirzad was the pick of the Afghanistan bowlers with 11 wickets.
In the fifth-sixth position play-off match, Sam Wood produced a fine all-round performance to help England defeat the West Indies by 13 runs.
Wood first hit a quick fire 104 off 111 balls with four fours and seven sixes to help England post 241 for nine and then picked up two wickets for 52 runs as the West Indies was dismissed for 228 in 48.3 overs. He was later declared player of the match. MORE