Chang Hua made the statement in an interview with ISNA, in an apparent reference to the United States' push to ban other countries from doing business with Iran.
Washington is making all-out efforts to reduce Iran's oil exports to zero as part of its "maximum pressure" policy that it adopted following its exit from the 2015 nuclear deal (formally known as the Joint Comprehensive Plan of Action) last year.
It initially granted waivers to eight major customers of Iranian crude, including China, but did not extend them after their May expiry, Financial Tribune reported.
However, Beijing, the largest importer of crude oil, remained a loyal customer and continued to receive oil shipments from Iran, defying US sanctions.
"China has always been opposed to the US unilateral sanctions … The government of Beijing will definitely protect the legal interests of Chinese companies," the ambassador said.
Chang described the project of redesigning the Arak heavy water reactor facility as an important feature of JCPOA, saying China has always worked to facilitate the plan's progress.
"Relevant authorities in China and Iran are constantly involved in the project and coordinate closely," he said, without giving further details due to the confidentiality of the project.
Under JCPOA, Iran agreed to redesign the Arak research reactor to cut its potential output of plutonium. Washington and Beijing had initially formed a working group to assist Iran in redesigning the Arak facility. Britain later replaced the US after it abandoned the deal.
As other signatories, particularly the European Union, have been unable to make up the economic damage inflicted by the sweeping US sanctions, Iran initiated a reciprocal plan by gradually lowering its commitments under the deal.
Officials in Tehran have declared that all moves would be reversed, as soon as Iran's rights under the deal are upheld.
Need to Restore JCPOA Balance
China believes that all parties to JCPOA need to fully meet their obligations and create a balance between countries' commitments and interests, the Chinese diplomat said.
The Tehran-Washington confrontation has triggered tensions in the Persian Gulf, which escalated after several attacks were made on oil tankers near the Strait of Hormuz in recent months.
The US blamed the incidents on Iran without any evidence and called for the launch of an international coalition to protect shipping lines in the strategic waters.
Iranian officials have described it as a threat, saying Tehran would not tolerate any aggression against its national security.
"In China's view, the main destabilizing factor in the region is the US unilateral exit from JCPOA," Chang said.
The Chinese ambassador called on both sides to refrain from taking any measure that would deteriorate the situation.