Abstract: The debris flow fans morphology is an integrated impact of its environment conditions. All of these conditions influencing the debris flow fans in the mountain area are sorted as inner-drainage basin and deposition basin. The way each condition influences the debris flow fans is analyzed systematically. Drainage basin area and channel gradient among the drainage factors, and width of main river valley and main river energy among the deposition basin factors, are elicited as more strong factors influencing the formation of debris flow fans. Using TM image and 1:50000 topographic maps, each debris flow deposited scopes are examined. Based on that, the relationship between the two most important factors of the inner-drainage basin and the debris flow fans is discussed. In the Xiaojiang River valley, the area of debris flow fans is basically increasing with the increasing of drainage area and a positive power function can be used to express the relationship; and a negative power function can be used to express the relationship between channel gradient and debris flow fan area. Lastly, the influence of depositional basin character on the Xiaojiang debris flow fans is mostly that the valid space confines the development of supper debris flow fans, like Jiangjia debris flow fan.