The invention discloses a debris flow diversion dam capable of automatically distributing drainage quantity and silt stopping quantity and a design method of the debris flow diversion dam. The debris flow diversion dam comprises a dam foundation and a dam body arranged on the dam foundation, diversion holes connected with a drainage channel are arranged at the bottom of the dam body, a diversion weir connected with a slit stopping field is arranged at the top of the dam body and positioned laterally above the diversion holes, the top elevation of the diversion holes is identical with the bottom elevation of the diversion weir, the size of the section of the diversion holes is determined according to the designed flow of the drainage channel, and the size of the section of the diversion weir is determined according to the designed flow of the slit stopping field. Compared with the prior art, the debris flow diversion dam capable of automatically distributing drainage quantity and silt stopping quantity and the design method of the debris flow diversion dam have the advantages that the diversion holes are fully utilized to control flow of debris flow entering the drainage channel, so that the debris flow is reasonably and efficiently distributed to the drainage channel and the slit stopping field, debris flow exceeding a designed standard is prevented from spillover in the drainage channel to threaten protected objects in the downstream, and the debris flow exceeding the designed standard is also prevented from flowing into a main river to form a dammed lake to cause secondary disasters.