The present invention relates to a calculation method for a dynamic reserve scale of a debris flow material source, and specifically to a debris flow material source dynamic reserve calculation method under a rainfall condition. The method comprises the steps of: according to plenty of field investigations and a debris flow material source type, establishing a geological model of material source destruction and conversion; and by using an on-site orthotopic artificial rainfall experiment, disclosing correlations about a conversion from slope destruction of a debris flow source region to a debris flow material source scale under different rainfall conditions, so as to establish a calculation model of a debris flow material source under a rainfall excitation condition. According to the method, proposed is the calculation method of carrying out calculation to determine a total dynamic reserve of the debris flow material source and, on this basis, calculating the dynamic reserve of the debris flow material source under the different rainfall excitation conditions, so as to provide solutions to key technical problems on geological exploration and project design such as a debris flow channel dam capacity and an exhaust conduit section.