Ashton will arrive in the Chinese capital Thursday from Tokyo, as she makes her first tour of the Asian giants since assuming her new office in December.
During visits to Beijing and then Shanghai, she will hold talks with Chinese President Hu Jintao, Premier Wen Jiabao and Foreign Minister Yan Jiechi among others.
"Iran is at the top of the agenda," the European diplomat said, as Western nations' attempts to put more pressure on Tehran to give up its nuclear programme struggle to translate into fresh UN sanctions.
The United States, Europe and others fear Tehran is seeking the technology to build nuclear weapons, while the authorities in Iran insist that their nuclear programme is purely for civil energy use.
China, a veto-wielding permanent UN Security Council member, is seen as key to the process by Europe.
Ashton, who built up relations with Beijing during her brief spell as EU trade commissioner, hopes the Chinese can be persuaded not to veto a move to further sanctions, even if they are not prepared to support it.
"The Chinese haven't supported the idea of fresh sanctions but they have allowed the process to move forward," the diplomat said.
"Probably the best we can hope for is an abstention," he said. "What we want to hear from the Chinese is that they are on the same page as the EU."
US Vice President Joe Biden said Thursday he expected "a sanctions regime coming out by the end of this month, the beginning of next month."
Ashton will visit China as part of an EU delegation with European Commission chief Jose Manuel Barroso.
"The EU and China are important global players and it is essential that we work together in addressing common challenges," Barroso said in a statement.
The EU delegation will also participate in the Shanghai World Expo 2010.
