You can fly into Mudanjiang from Seoul, Beijing, Shanghai, Shenyang, Guangzhou, Qingdao and Yantai.
Trains from Harbin take 4h30 to 6 hours and cost ¥25-75. There are also trains from Dalian (18-19 hours; ¥81-438), Beijing (21 hours; ¥171-516), Rizhao (34 hours; ¥234-706), Manzhouli (20 hours; ¥78-270), Shenyang (12-14 hours; ¥-), Qiqihaer (- hours; ¥-) and Suifenhe (- hours; ¥-).
From Suihenhe, Harbin. Also from Vladivostok, Ussuriysk, Pogranichniy (Russian Federation). For those coming from Changbaishan in Jilin province, there are daily buses from Changbaishan that pass through Dunhua to Mudanjiang.