In a brief statement dated Friday but reported by Chinese media on Saturday, the worldwide department said Song, who is there representing Xi, reported to North Korean official Choe Ryong Hae the outcome of the congress.
He is the first major Chinese envoy to visit Pyongyang in more than a year with ties severely strained between the two countries.
A lack of response from the North Korean leader toward an ongoing visit by China's special envoy may be a sign of Kim Jong-un's frustration toward China's newfound cooperation with the United States, an expert in South Korea said Monday.
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!".
The inspection was led by Choe Ryong-hae, the vice chairman of the Central Committee of the North's ruling Workers' Party.
"Song Tao informed in detail of the 19th National Congress of the CPC", the KCNA report said.
The information was disclosed by the NIS in a briefing to South Korea's parliament.
Trump has repeatedly suggested that China could easily solve the North Korea nuclear problem by tightening the screws on trade.
Bonnie Glaser, China specialist at the Center for Strategic and International Studies in Washington, told AFP that it was hard to unpick the true tenor of the trip.
"Relations are extremely stressed". They exchanged views about the situation on the Korean Peninsula and in the region, and bilateral relations, the KCNA said.
China sees it as a threat to its own military capabilities.
But Beijing fears that squeezing Pyongyang too hard would cause its collapse.
As Song wrapped up his trip, South Korea announced that Foreign Minister Kang Kyung-Wha would travel to Beijing on Tuesday to prepare for a December visit by President Moon Jae-In. It, however, didn't elaborate on what was discussed over the dinner table or if he would be able to meet Kim.
Song's department is in charge of the party's relations with foreign political parties, and has traditionally served as a conduit for Chinese diplomacy with North Korea.
Koh also said the North Korean strongman, as head of the rogue state, is being cautious of the message the meeting with Song could send to the global community.
Analysts had expected that Song would press North Korean officials on the nuclear standoff, but did not expect any breakthroughs.
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.