In order to ascertain the numbers of the electrodes (treatment for the sewage) and forecast the grainsize property of the cracked mineral, based on the hypothesis of the equivalent effective volumes of one discharge of electrohydraulic power impulse technology, the formulas are worked out for computing the effective working areas of the two common working modes. The theoretic analysis and the results of the experiments of the treatment of bacillus coil liquid and crushing of quartz ceramic show that the results conform to the formulas relatively, and the precision of the calculated data has relations with the polluted degree of the sewage, the intensity, elasticity and dimension of the solid material, and the formulas for computing the effective working areas are feasible.