Donald Trump is warned "there is no room for bargaining" over the "One China" policy as Taiwan's president meets US officials.
Beijing may "take revenge" if the US drops its "One China" policy, a state-run Chinese newspaper has said.
The warning comes just hours after Taiwanese President Tsai Ing-wen met senator Ted Cruz in Houston, Texas.
Following the meeting, former presidential candidate Mr Cruz said China "needs to understand that in America we make decisions about meeting with visitors for ourselves".
The comments have reignited fears in Beijing that President-elect Donald Trump may formally recognise Taiwan, which is considered by China as a rogue province awaiting unification.
The Global Times, a newspaper published by the ruling Communist Party, said in an editorial that Washington's long-standing "One China" policy was "an obligation of US presidents to maintain China-US relations and respect the existing order of the Asia-Pacific".
It added: "If Trump reneges on the 'One China' policy after taking office, the Chinese people will demand the government to take revenge. There is no room for bargaining."
China foreign ministry spokesman Lu Kang said Beijing was "firmly opposed" to the meeting between Ms Tsai and Mr Cruz.
He claimed it had been part of Ms Tsai's "attempt to undermine China-US relations"
Mr Cruz said he and Ms Tsai discussed improving bilateral relations and furthering economic co-operation between Taipei and Washington.
During a stopover while on the way to Central America, Ms Tsai also held talks with Texas governor Gregg Abbott, who tweeted a picture of the pair sitting by a table adorned with US and Taiwanese flags.
According to Ms Tsai's office, the Taiwanese president also spoke by phone with senator John McCain, head of the senate committee on armed services.
Last month, Mr Trump sparked fury in China by speaking with Ms Tsai over the phone.
Today I met with the President of Taiwan to discuss expanding trade and economic opportunities. #txlege
Vice President-elect Mike Pence dismissed the discussion as "nothing more than a courtesy call" but Beijing said it was "seriously concerned"about Mr Trump's views on Taiwan.
Relations between Mr Trump and Beijing were dealt another blow last month after the President-elect criticised the Chinese seizure of a US underwater drone in international waters.
Mr Trump misspelled the word "unprecedented" as he conveyed his outrage over the move on Twitter.