Li attends a two-day summit in Bangkok of leaders of Mekong River region countries - Cambodia, Laos, Myanmar, Thailand and Vietnam - the biggest international gathering in Thailand since its military seized power.
Li's presence will offer reassurance to a junta seeking legitimacy after some Western countries imposed sanctions and downgraded ties in response to the coup, and will provide a counterbalance to a tougher US stand toward its oldest regional ally.
Thai government spokesman Yongyuth Mayalarp said the summit showed the international community's renewed confidence after months of unrest and said Thailand and China were "great friends in trade".
"It's a good opportunity for Thailand to show that our political problems are not an obstacle to trade," Yongyuth told reporters.
"The situation here is normal now and we are working toward a new Thai democracy."
The army seized control on May 22 after months of protests against the government of populist Prime Minister Yingluck Shinawatra. It said the coup was necessary to restore stability after 30 people were killed in sporadic violence.
Thai Prime Minister Prayuth Chan-ocha, the general who led the coup, will preside over the alliance of countries ruled by communists, former communists and retired soldiers, which the United States and China have been eager to engage.
Li's presence underscores China's growing influence in Southeast Asia, where it has bolstered ties with a raft of loans and aid for infrastructure.
A Beijing-based diplomat who follows China's regional engagement, and spoke on condition of anonymity, said Thailand was a "big potential U.S. ally for China to win over post-coup".
Prayuth also visits Beijing next week.
Li and Prayuth will sign memorandums of understanding at the Greater Mekong Subregion summit on Sino-Thai rail links and rice, said Yongyuth, without giving details.
Thailand is seeking buyers for a record 17 million tonne stockpile of rice built up under the previous government's state-buying programme.
Li is unlikely to publicly address sensitive issues such as Thai political prospects or Muslim Uighur people who have been trickling out of China's violence-prone western region of Xinjiang to Southeast Asia.