A debris flow prediction model is often a statistical model which mainly uses the information of rainfall as the single basis，resulting substantial inaccuracy and indeterminacy in the forecast． Debris flows occur when the soil mass lost its stability owing to the pressure of surface runoff and the lubrication of the infiltrated water． The saturated soil depth by infiltrated rainfall can therefore be used as the lower boundary of debris flow source to enhance the physical elements of debris flow forecast． We described the working principles and the structure design of the detector for saturated soil depth． The technique was then employed to obtain the depth of rainfall saturated soil in the source of the debris flow． In addition to the real-time rainfall data，the saturated soil depth was used to forecast the occurrence of debris flow． According to the field experiment and the instrument observations，the accuracy of debris flow forecasting can be improved by monitoring the rainfall infiltration depth in soil of the debris flow source．