Lew's visit to Beijing on Tuesday and Wednesday comes at a crucial time, the official told reporters on condition of anonymity. "China is undergoing their once-in-a-decade leadership transition and, of course, their reform process is at a crossroads."
"It's important to deepen our relationship with China's new leadership team at this time," the official said.
Lew lacks the international stature of his predecessor, Timothy Geithner, and is signaling the importance the United States put on its economic relationship with China by making his first international trip as secretary to Beijing.
Secretary of State John Kerry in coming weeks will also make his first trip to China since taking office last month.
Both Kerry and Lew will host their Chinese counterparts in Washington in the middle of this year for the annual U.S-China Strategic and Economic Dialogue, the official said.
China's legislature formally chose Li Keqiang as premier on Friday, installing the English-speaking bureaucrat as the man in charge of the world's second-largest economy.
President Barack Obama raised U.S. concerns about computer hacking in a phone call with Xi on Thursday, just days after U.S. intelligence leaders said for the first time that cyber attacks and cyber espionage had supplanted terrorism as the top security threat facing the United States.
"We will press China to take serious steps to investigate and put a stop to these activities and to engage with us in a constructive direct dialogue to establish acceptable norms of behavior in cyberspace, recognizing it is a growing challenge for both of us," the senior U.S. official said.
Lew will also press China to allow its currency to rise further against the dollar and push on other concerns such as increased market access for U.S. goods and better protection of intellectual property rights, the official said.
China's yuan has appreciated 16 percent in real terms against the dollar since June 2010. "More progress, however, is needed," the official said.
(Reporting by Doug Palmer; Editing by Doina Chiacu)