A roof will be placed on the tank, which will hold more than 60,000 cubic metres of gas.
The work is being done by the plant's owner Ineos to hold imported shale gas from the US, obtained through the controversial fracking process.
The development of unconventional gas extraction has been halted by a Scottish government moratorium.
Ineos announced plans last year to invest up to Â£640m in shale gas exploration in the UK and use it as a raw material for its chemicals plants.
The company hopes they will eventually be able to use gas that has been fracked locally in future rather than relying on stocks from the US.
The new storage tank is 56m (183ft) in diameter and 44m (144ft) high - 560 double decker buses could fit inside its 108,372 cubic metre interior.
The 330 tonne roof of the tank was lifted into place using a cushion of air similar to a hovercraft.
John McNally, chief executive of Ineos Olefins and Polymers UK, said it was a "landmark day for Grangemouth".
He said: "We know that US ethane has transformed US manufacturing and now Scottish industry will benefit as well.
"This will secure a cost-effective supply of ethane for the next 15 years, and give a sustainable base for Grangemouth for that time."
Ineos has They have started a community consultation process to try to win support for fracking.
The firm said it was unconcerned about the moratorium and a spokesman promised to drink "a lot of tea in a lot of village halls" to try to win the argument.
However, Friends of the Earth Scotland director Dr Richard Dixon said: "We strongly suspect Ineos' plans for fracking in Scotland would result in blighted communities, with planning permissions sitting unused for years.
"The last thing anyone wants is to have a site next to their home where fracking may happen at some unspecified time in the future.
"Communities could have to wait until nearly 2030 to find out if they are going to be fracked or not.  This threat of blight is another good reason for the Scottish government to decide that fracking has no place in Scotland."
Work to complete the biggest shale gas tank in Europe is continuing at the petrochemical plant in Grangemouth.