Abstract: How to determine the digging depth is the key factor and also a problem which is often difficult to be tackled in determining the digging resistance for the ore piles and the piles after blasting. Based on the typical digging process for a blasted ore pile and the principle that the volume digged by a shovel dipper is equal to the volume of the dipper, a mathematical model is developed of digging depth with an equation of digging path given. Then, the mathematical relationship between the maximum digging depth and inside width of dipper is given on the extreme principle, which is in conformity to the measured values. Furthermore, the digging resistance and its maximum value in the whole digging process are both given. In this way a new method is proposed to determine the key factor value of digging resistance, thus providing a reliable reference for determining other mechanical parameters.