The Tianchi volcano, Changbai Mountain,being one of the most famous volcanoes in the world, is an active volcano with the largest scale and potential danger in modern history.But its accurate eruption time has not been confirmed by the historical records.By collecting,consulting,arrangging, textual researching and analyzing the huge amount of historical records, it is concluded that the Tianchi volcano,Changbai Mountains, erupted intensively during 1199～1201 AD.