The remarks by China's Taiwan Affairs Office came two weeks before Tsai Ing-wen, chairwoman of the Democratic Progressive Party (DPP) and a presidential contender in January elections, meets government officials, academics and overseas Taiwanese in the United States.
The United States has no formal diplomatic ties with Taiwan but is the democratic island's biggest ally and provider of weapons.
Previous trips by senior Taiwanese politicians to the United States have angered Beijing and damaged Sino-US relations.
"We firmly oppose any person engaging in any form of 'Taiwan independence' separatist activities in the international arena," Taiwan Affairs Office spokeswoman Fan Liqing told a regular news briefing in response to a question about Tsai's 12-day trip.
Fan said the peaceful development of cross-strait relations depends on opposing Taiwan independence and on the "1992 consensus", referring to Beijing's cherished "one China" principle that includes Taiwan as part of China.
Tsai has said she favors "maintaining the status quo" when asked about her China policy, Reuters reported.
China regards Taiwan as a renegade province, to be put under Beijing's control by force if necessary.
Beijing last month warned the DPP to heed the lessons of the last time it was in power and not push for independence.
The DPP's Chen Shui-bian infuriated Beijing and strained Taiwan's relationship with the United States during his time as president from 2000 to 2008.
China accused him of trying to push for independence and weaken the island's Chinese cultural heritage.