"The free trade agreement is very one-sided... the numbers don't match," ex-president Mohamed Nasheed told the BBC.
The Maldives' parliament would not approve the law required for the trade deal to come into force, he added.
The comments come days after Mr Nasheed's ally Ibrahim Mohamed Solih became the Maldives' new president.
Mr Nasheed - a senior advisor to President Solih - was barred from contesting the September elections.
There has been no immediate response from China to the latest comments, but a week ago the Chinese embassy in the capital Male rejected a statement by Mr Nasheed in which he said his country risked falling into a debt trap with Beijing.
The previous president, Abdulla Yameen, preferred close links with China and signed the free trade agreement during a visit to Beijing last December.
China and Chinese companies have invested hundreds of millions of dollars in various infrastructure projects in the Indian Ocean nation, known for its luxury resorts and coral islands.
But Mr Nasheed's comments are seen as the latest sign of a backlash against Beijing in the Maldives.
The Indian-Ocean archipelago is among a number of small countries where China has invested billions of dollars building highways and ports as part of its Belt and Road Initiative, to promote trade between Asia and parts of the rest of the world.
Mr Nasheed also said China and Chinese companies have taken over a number of islands in the Maldives on leases ranging from 50 to 100 years.
"If any of these investments (in the islands) are not commercial then we must review it… Nothing was made public in the last five years," the former president said.
Mr Nasheed prefers close ties with India, which has been trying to regain its influence in the Maldives.
The Indian prime minister, Narendra Modi, was among the guests who attended the swearing-in of President Solih in the capital Male on Saturday.
Mr Solih is also expected to make his first state visit to India next month.