Until now，the formation process of debris flow from natural dam failure is still unclear and there has not been a formation condition for outburst debris flow. In this paper，we present the results of a series of laboratory tests that assessed three different materials，five different flume bed slope angles (2°，7°，9°，10°and 13°)，two in-flow rates and four types of dam geometric shapes. It shows that debris flow is easier to form from natural dam failure than gully erosion. The unit weight of water-sand mixture in the downstream increases firstly，and then decreases with time developing. The results also show that the unit weight of water-sand mixture is seriously affected by channel bed slopes. In addition，the decrease of median diameter and the increase of downstream slope can both increase the unit weight of water-sand mixture，and the unit weight of water-sand mixture is less sensitive to downstream slope of the dam than channel bed slopes and dam geometric shapes. Based on the theory of stream power and experimental data，we established a critical condition for debris flow formation from natural dam failure，which is simple to calculate and the parameters can be obtained easily. Combining four cases，it indicates that the established critical condition agrees well with the field.