May arrived in the central industrial city of Wuhan and will be in the country until Friday in what the Chinese foreign ministry has called a "historic visit".
She was heading to Beijing later Wednesday to meet Premier Li Keqiang.
"My visit will intensify the 'Golden Era' in UK-China relations. The depth of our relationship means we can have frank discussions on all issues," she said earlier this week.
May is battling criticism of her Brexit strategy back home. The House of Lords is scrutinising a key piece of legislation on quitting the EU as a leaked government report shows only economic downsides to leaving the bloc.
China "is delivering a new source of capital that --with the appropriate safeguards in place -- can help us to invest in the future of our country," she wrote in a column in the Financial Times.
"Together, that means a stronger British economy and more and better jobs for British workers."
May is accompanied by her husband Philip and a delegation of 50 business leaders and organisation representatives, which her office said was "the largest" Britain has ever taken overseas.
China also has high expectations that London will endorse its Belt and Road Initiative (BRI), a massive infrastructure project aimed at reviving ancient Silk Road trade routes between the East and West and creating greater market access for Chinese companies.
"It's natural that Belt and Road cooperation is an opportunity for the two sides to tap into our cooperation for win-win results," Chinese foreign ministry spokeswoman Hua Chunying said on Tuesday.
She will hold talks with President Xi Jinping on Thursday and finish her visit Friday in the eastern business hub of Shanghai.
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