Earlier Wen Jiabao, the Chinese premier, said advancing six-party nuclear talks was a key issue, and called North Korea a "friendly neighbour".
China is one of North Korea's closest allies and most important trading partners, and also plays host to six-nation talks aimed at dismantling North Korea's nuclear programmes.
In an interview with China's official People's Daily, Kim, who is not related to the North Korean leader, Kim Jong-il, described Beijing as a diplomatic anchor for his country amid international turbulence.
"North Korea feels satisfied with the vigorously friendly relations with China during the current complex and changeable international trends," he said in a written reply to questions from the newspaper before his trip.
He wrote that the bond would "make a real contribution to ensuring the peace and stability of the Korean peninsula, the northeast Asia region and the world".
In recent diplomatic exchanges, US and Japanese officials have said that China is opposed to the missile launch but Beijing has not publicly expressed any direct opposition.
Also on Tuesday, officials in Seoul said North Korea had fully reopened its border to South Korean goods and staff heading to a jointly-run industrial park on the North's side of the heavily-fortified frontier.
"North Korea informed us overnight that all bans on crossings will be completely lifted today," Yoo Chang-Geun, vice-chairman of the Corporation of Kaesong Industrial Council, told the AFP news agency.
North Korea had partially reopened the border crossing, allowing workers at the Kaesong industrial zone to return home but not allowing traffic in the other direction.
Pyongyang had closed the border on Friday for the second time in less than a week in an apparent protest against joint US-South Korean military exercises.
The closure had left hundreds of South Koreans and 200 trucks stranded on the northern side of the border.
Lee Jong-Joo, a spokeswoman for Seoul's unification ministry, confirmed on Tuesday that the North's military had sent a letter authorising all South Korean trips via the western border crossing.
She said 546 South Koreans were scheduled to travel northwards while 307 would be returning to the south on Tuesday.
Some 38,000 North Koreans work in about 100 South Korean-owned firms in Kaesong, producing goods such as watches, clothes, shoes and kitchenware.
Raw materials are trucked northwards into the industrial estate while the finished products travel the opposite way.