Chinese Foreign Minister Yang Jiechi arrived in Phnom Penh Thursday for an official three-day visit, in response to an invitation from Foreign Minister Hor Namhong.
Cambodian officials called the visit a celebration of a 50-year diplomatic relationship between the two.
Opposition leaders said the Chinese government should help fund the Khmer Rouge tribunal.
"Because China once helped the Khmer Rouge, who made us suffer, if China wanted to help the Cambodians, it would donate so that those who have passed away and those who are still alive get justice," said Eng Chhai Eang, secretary-general of the Sam Rainsy Party.
"If China was interested in donating money for the Khmer Rouge trial, it would be great," said Keo Remy, vice president of the Human Rights Party.