The runoff data measured from 1978 to 2009 at Zhimenda hydrological station in the headwaters of Yangtze River are analyzed by Mann-Kendall test and slide T test to study the trend of runoff variation. Impacting factors including precipitation, temperature, and underlying surface condition are also elaborated in this paper. The results show that the overall annual runoff increased slightly during 1978-2009 ； while there was an abrupt change around 2004 according to slide T test. With regard to the impacting factors, the annual precipitation witnessed slight changes in general and a sudden change around 2002, and the annual air temperature series within the basin increased remarkably. It is concluded that the runoff increase during 1978-2009 is mainly caused by the increase of precipitation and ice melting.