Board member Buntoeng Vongkusolkit said cane-crushing capacity would also more than double to 1 million tonnes.
Of the 3.5 billion baht, 2.5 billion will go to developing production facilities and 1 billion to farm plantation.
The additional capacity will be fed by a plantation area located near the existing production facility in Laos's southern province of Savannakhet.
The new capacity is expected to start up at the 2019-20 crop, with the crushing period starting in November 2019 and running through April 2020.
"Actually, the doubled capacity should have begun last year, but the programme was delayed because of an outbreak of white leaf disease over the past three years," Mr Buntoeng said.
Mitr Phol is confident that the disease will be completely wiped out by the end of this year.
Once the capacity increase starts, sugar cane from local planters will rise from 60% to 70%.
"This programme aims to create jobs for local people by reducing our plantation area," Mr Buntoeng said.
In preparation for the capacity expansion, the company will study organic sugar production to add value.
The project includes a new biomass power plant with a capacity of 6-10 megawatts, adding to the 9 MW generated by an existing biomass plant.
The company will talk to local authorities about possibly selling the additional electricity to Laos.
Mitr Phol branched into Laos in February 2006 with its wholly owned Mitr Lao Sugar Co.
The company obtained a 40-year concession for a sugar-cane plantation area spanning 62,500 rai.
After a 2.3-billion-baht investment, the first crop of sugar cane arrived a year later.
Up to 80% of sugar production in Laos is exported, while the rest is for domestic consumption.
The major sugar makers in Laos are two Thai companies — Mitr Phol and Khon Kaen Sugar Industry Plc — and a Vietnamese firm, Hoang Anh Gia Lai.