The invention provides a flow velocity and flow quantity calculating method of debris flow passing through a grid-type dam. The flow velocity and flow quantity calculating method comprises the steps that the volume-weight of the debris flow at the upstream and the downstream of the grid-type dam, the characteristic particle size of the debris flow, the width of a ditch, the opening space of the grid-type dam, the diameters of crossbeams and vertical beams of the grid-type dam and the number of the crossbeams of the grid-type dam are obtained; the flow velocity coefficient and the flow quantitycoefficient are rated through a coefficient rating formula obtained by combining basic parameters obtained through field study with an indoor model test; and the flow velocity and the flow quantity of the debris flow passing through the grid-type dam are calculated. The flow velocity and flow quantity calculating method of the debris flow passing through the grid-type dam has the beneficial effects that the property of the debris flow, dam body parameters and a ditch condition are taken into consideration comprehensively, the velocity coefficients and the flow quantity coefficients under different conditions are rated by combining the indoor model test, and the velocity and flow quantity calculating formulas are obtained through theoretical derivation, so that the basis is provided for application of debris flow prevention and treatment engineering.