It is not clear whether Song will meet North Korea's youthful leader Kim Jong Un.
The ruling Communist Party of China (CPC) is ready to "steadily develop traditional friendly relations" with North Korea, said Beijing's top envoy who is on a visit to Pyongyang.
The North's state-run Korean Central News Agency (KCNA) released a statement on Saturday, 18 November, detailing Song's visit to Pyongyang.
China has yet to offer any details of Song's visit, which began on Friday, and it is unclear how long he will be in North Korea.
Analysts expect Song to address the nuclear standoff, which has roiled relations between the two Cold War-era allies as China has backed United Nations sanctions on North Korea over its missile tests and sixth nuclear blast.
"They said that the traditional friendship between China and North Korea was founded and cultivated by both countries former old leaders, and is valuable wealth for the two peoples", the department said.
The visit is "more about warning North Korea against going to extremes than forcing it to give up the nukes" because China's top priority remains maintaining regional stability, he said.
North Korea has not conducted any missile tests since September 15, raising cautious hopes in the region that the country might be ready to ease tensions.
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But experts doubt Song's visit will yield major breakthroughs.
Song also traveled to Hoechang County, west of Pyongyang, to tour what was Chinese military forces' headquarters during the 1950-53 Korean War, it also said. China sends a special envoy to Pyongyang to explain the results of its party congress, held every five years, as it did in 2007 and 2012. Mr. Song was expected to convey a message from Mr. Xi to North Korea to join negotiations to halt its nuclear programme.
The two sides "exchanged their views on such matters of mutual concern as the situation of the Korean Peninsula and region and bilateral relations", reported the KCNA that day.
On Saturday, Song held talks with former Foreign Minister Ri Su-yong, a vice chairman of the Central Committee of the North's Workers' Party.
"The key to easing the situation on the peninsula lies in the hands of Washington and Pyongyang".
Trump, who warned Xi during his trip to Beijing last week that time was "quickly running out" to solve the nuclear crisis, took to Twitter on Thursday to hail the mission as "a big move, we'll see what happens!" "Perhaps the lowest point since the Korean War", she said.
On North Korea refraining from testing a ballistic missile over the past two months, Mattis said, "We're watching it closely", without confirming if this was a positive signal.