The classy 31-year-old, who fell three short of his first Sharjah hundred against Pakistan on Friday, smashed four sixes and 11 boundaries after coming in at 11 for two in the sixth over.
De Silva's eighth one-day hundred lifted Sri Lanka to an imposing 251 for seven, and Pakistan were bowled out for 200 despite a power-packed 62 off 66 balls by the burly Inzamam-ul Haq.
Sri Lanka's third successive win in the double-leg preliminary league ensured their place in the final, leaving Pakistan and winless Zimbabwe to fight for the second spot on Wednesday.
De Silva, who made 60 and 97 in the previous two games for an aggregate of 291 runs so far, earned his third successive man of the match award.
Starting with a six over square leg off paceman Waqar Younis, he reached his century in the 48th over by sweeping off-spinner Saqlain Mushtaq for his ninth boundary.
He celebrated the feat with a six and four off Younis, and then smashed two more sixes in Saqlain's final over before being caught at point off the last delivery.
Pakistan were reduced to 49 for three by the 13th over, before Inzamam and Rameez Raja (47) gave their team hope by adding 77 for the fourth wicket.
But Sri Lanka's three-pronged spin attack, led by off-spinner Muttiah Muralitharan, ripped through the middle-order to swing the match their way.
Muralitharan, who earlier took an amazing catch off a full-blooded shot from former Pakistan captain Salim Malik, had both Raja and Inzamam caught in the deep with well-disguised slower balls.
Off-spinner Kumara Dharmasena chipped in with the wickets of Moin Khan and the dangerous Wasim Akram (27 off 25 balls), before sealing Sri Lanka's triumph by removing last man Younis.
Earlier, Younis had given Pakistan a headstart by trapping Romesh Kaluwitharna and Marvan Atapattu leg-before in his second and third overs.
Sanath Jayasuriya (67) and De Silva counter-attacked with a brisk partnership of 124 off 139 balls to carry Sri Lanka to 135 for two.
Part-time bowler Salim Malik then applied the breaks with the wickets of Jayasuriya and skipper Arjuna Ranatunga in his second over, the 29th of the innings.
Sri Lanka were tied down to 37 runs between the 31st and 40th overs by leg-spinner Shahid Afridi who conceded just 30 runs in an unbroken 10-over spell.
Afridi earned a dubious verdict for a catch at the wicket against Roshan Mahanama (12) and Sri Lanka slipped further back when Saqlain forced Hashan Tillekaratne (13) to give an easy return catch.
But Kumara Dharmasena (10 not out) helped De Silva survive the final overs as Sri Lanka piled up the highest score in the tournament so far.