Cheng, chairwoman of Taiwan's largest opposition party the Kuomintang (KMT), is in China on what she calls a "peace" mission to lessen tensions at a time when Beijing has stepped up military pressure against the island it calls its own.
China refuses to talk to Taiwan President Lai Ching-te, saying he is a "separatist".
Lai's administration has called on Cheng to tell China to stop its threats, and says Beijing should engage with the democratically elected government in Taipei.
Speaking to reporters at Shanghai's Yangshan Port on Thursday, Cheng said she was fond of how ancient Norse sailors described the sea as the "road of the whale".
"These words are spoken with such humility, and they are entirely right. What should fly in the sky are birds, not missiles.
What should swim in the water are fish, not warships," she said, in comments carried live on Taiwanese television stations.
Cheng, who flies to Beijing later on Thursday for a possible meeting with Chinese President Xi Jinping, also quoted, in English, part of John McCrae's World War One poem In Flanders Fields: "If ye break faith with us who die, We shall not sleep."
"We may not have been able to give our ancestors peace, but we can certainly still give peace to the people of today and the people of the future," she said.
In Taipei, MPs from the ruling Democratic Progressive Party (DPP) condemned the KMT for skipping talks in parliament on how to progress with stalled plans to spend an extra $US40 billion ($A57 billion) on defence.
"Don't intentionally put this off because of the meeting with Xi Jinping tomorrow. Don't link this to the Chinese communists," said the DPP's Chen Kuan-ting, who is joint head of the defence and foreign affairs committee.
Neither China nor the KMT has confirmed Cheng is meeting Xi on Friday.
The KMT, which says it supports the defence spending but will not sign "blank cheques", said in response that Cheng's "peace visit to the mainland has no connection whatsoever with the special defence budget".
Beijing has also maintained its daily military activities around Taiwan despite Cheng's presence in China.
Taiwan's defence ministry said on Thursday morning that during the previous 24-hour period it had detected six Chinese military aircraft and eight warships around the island.
"The facts prove that the Chinese communists' military threat against Taiwan is intensifying," DPP spokesperson and MP Michelle Lin wrote on her Facebook page.
"Cheng Li-wun has been on her trip for two days, and the Chinese communists still have a knife at Taiwan's throat."
Lai has repeatedly offered talks with Beijing, whose sovereignty claims he rejects, saying only the island's people can decide their future.
