"The speaker has the right to visit Taiwan," he told reporters.
"There is no reason for Beijing to turn a potential visit consistent with longstanding US policies into some sort of crisis."
Kirby cited intelligence that China was preparing possible military provocations that could include firing missiles in the Taiwan Strait or "large-scale" incursions into Taiwanese airspace.
He said Pelosi was travelling on a military aircraft and that while Washington does not fear a direct attack, it "raises the stakes of a miscalculation".
Kirby reiterated, however, that US policy was unchanged toward Taiwan.
This means support for its self-ruling government, while diplomatically recognising Beijing over Taipei and opposing a formal independence declaration by Taiwan or a forceful takeover by China.
Taiwan's government has remained silent on the prospect of a Pelosi visit. Prime minister Su Tseng-chang did not confirm the visit on Tuesday when asked by reporters but thanked Pelosi for her support.
And Taiwanese newspaper Liberty Times cited unnamed sources as saying Pelosi would land on the island Tuesday night, then meet Tsai the next day before departing in the afternoon.
More warnings from China
Taiwan's 23 million people have long lived with the possibility of an invasion, but the threat has intensified under Chinese president Xi Jinping.
In a call with Biden last week, Xi warned the United States against "playing with fire" on Taiwan.
And on Monday, China's ambassador to the United Nations, Zhang Hun, said such a visit would be "very much dangerous, very much provocative".
If it happens, "China will take firm and strong measures to safeguard our sovereignty and territorial integrity", he said.
American officials often make discreet visits to the island to show support, but a Pelosi trip would be higher-profile than any in recent history.