This is called "Tán Yuè", literally "playing music" or "plucked string music", but apparently actually means pingtan music or a tune of pingtan. Pingtan is a type of storytelling music from Suzhou in Jiangsu province in China.
This erhu piece is composed and played by Sun Wenming (孙文明, 1928-1962).
This piece is unique in the erhu reportoire, as it is played, apparently, on an erhu without a qianjin (a loop of string which acts as a nut). According to a CD liner notes, it's influenced by the playing style of the sanxian (3 string lute) used in Pingtan.
tán yuè 弹乐 A Tune of Pingtan - Sūn Wénmíng 孙文明.mp3, 4MB mp3 file
Played by Sun Huang:
Played by Song Fei: