"Mishandling the issue is not conducive to strategic stability in the region, and could intensify disputes," Xinhua news agency quoted Xi as saying on the sidelines of the second day of the ongoing G20 summit here.
China has repeatedly expressed "deep concern" since the US and South Korea decided in July to deploy the US Terminal High Altitude Area Defence (THAAD) system to counter missile and nuclear threats from North Korea.
China and South Korea should put bilateral ties on the right track toward stable development, Xi said.
The two countries should cherish their existing cooperative foundation and overcome difficulties and challenges, Xi said in a meeting with President Park.
The two countries are "close neighbours with broad common interests", Xi said.
Park said the South Korean government has the firm will to value and develop its bilateral relations.
In response, North Korea has threatened a "physical response" against the THAAD decision.