The invention discloses a debris flow rainfall I-D curve threshold construction method and a debris flow early warning method. The I-D curve threshold construction method comprises the following stepsof firstly, acquiring basin underlying surface data required for a Richard model and Van Genuchten equation numerical simulation; starting a hydrologic simulation process under a given rainfall intensity condition; determining the stability and the runoff generation state of each grid in a drainage basin along a time sequence, and calculating the bulk density of a water and soil mixture; and recording data according with a control point to fit an I-D curve, and finally acquiring a set of I-D thresholds under a previous rainfall amount condition. The invention simultaneously provides a corresponding drainage basin debris flow early warning method. By using the method of the invention, the idea of fitting the curve through historical data statistical analysis is abandoned, and the breakthrough of constructing a debris flow generation condition threshold from the aspect of the measurement of the dynamic index of a debris flow characteristic is realized. The method can effectively solve the problem of using the I-D curve threshold to implement debris flow early warning in the drainage basin without data.