The ring-shaped reef, 250 kilometres from the Philippines, has been claimed and occupied by China since 1995.
The satellite images show building work in seven areas between May 2020 and February 2021.
One image dated May 7, 2020 shows an empty plot of land, which is now occupied by a 16 metre-wide cylindrical structure.
According to Simularity, it could be a “possible antennae mount structure”.
Another image shows a concrete structure with a weatherproof enclosure used to protect a radar antennae nearby.
Simularity said this could “possibly be a fixed radar structure”.
Other sites on Mischief Reef appear in mid-construction or have been cleared for development.
At least six other governments also have overlapping territorial claims in the contested waterway: the Philippines, Vietnam, Malaysia, Indonesia, Brunei and Taiwan.
Trade shipments worth trillions of dollars pass through the South China Sea every year.
Mischief Reef has been at the centre of tensions between China and the Philippines.
In 2016, the Permanent Court of Arbitration in The Hague ruled that it is within the exclusive economic zone of the Philippines.
Relations between China and the Philippines remain strained.
Last month, China passed a law that gave its navy and coast guard the power to take all necessary measures to defend its sovereignty in the disputed waters.