The army said it had originally estimated mine-infested areas totalled some 6,215 square kilometres (2,399 square miles) in the island's embattled northeast.
But later surveys showed the mined areas covered about 2,046 square kilometres.
"All agencies involved in de-mining in Sri Lanka were able to clear 1,912 square kilometres (by September 2011) and the remaining contaminated area is now restricted to about 133 square kilometres," the army said in a statement.
The military stepped up its mine clearing operations after defeating the separatist Tamil Tiger rebels in a major offensive in May 2009.
The separatist conflict, which began in 1972, cost around 100,000 lives, according to the UN.
The army said remaining mine-infested areas had a "very high density of mines, traps and improvised explosive devices.
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Sri Lanka's army and eight other groups are involved in the mine removal efforts.