Liu enrolled in a junior sports school in fourth grade and initially succeeded at the high jump. He switched to the hurdles at age 15 and debuted internationally at the world junior championships in 2000, finishing fourth in the 110-metre event. He won the same race at the 2001 World University Games and in 2002 set world junior records indoors in the 60-metre hurdles (7.55 sec) and outdoors in the 110-metre hurdles (13.12 sec). In 2003 he earned bronze medals at the indoor and outdoor world championships and was named his country's Male Athlete of the Year. Liu won the 110-metre hurdles at the 2004 Olympic Games in Athens, equaling the world record of 12.91 sec and becoming just the sixth man to post a time under 13.00 sec. In 2006 he set a world record (12.88 sec) in the event.