He said Beijing already had a three-stage invasion plan in place and is expected to designate an "air defence identification zone" and open a civil air route along the Taiwan Strait.
"In doing so, China is once again challenging and attempting to unilaterally change the status quo in the Taiwan Strait."
Chen said that when he took office in 2000, China had only about 200 missiles aimed at the island.
His comments came in what will be his final new year address as president ahead of elections due in March.
Chen, who has already served two four-year terms, is constitutionally barred from seeking re-election.
Taiwan and mainland China split at the end of the Chinese civil war when communist forces took control of the mainland.
The island has been self-ruled ever since but Beijing continues to claim sovereignty over it, threatening war if Taiwan formally moves towards independence.
In his address, Chen also defended plans to hold a referendum on a bid for United Nations membership under the name "Taiwan" alongside presidential elections in March.
|Taiwan regularly hold exercises preparing |
for an invasion from the mainland [EPA]
The plan has angered both Beijing and Washington, the island's key ally which says the issue risks inflaming tensions with the mainland unnecessarily.
Chen said the referendum was "an expression of our people's will" and "a basic right guaranteed by law".
"The referendum is a grass-roots initiative", he said, that "cannot be opposed or cancelled by anyone, not even the president."
Chen said the US was caving into Chinese pressure and disregarding democratic values by opposing the referendum.
The US fears that if conflict were to break out between the mainland and Taiwan, it could be drawn into the fighting and has urged Chen against taking actions seen as provocative to Beijing.