In order to propose a signal processing method based on harmonic analysis suitable for different media and working conditions, it is necessary to reselect a higher-order harmonic as characteristic variable linearly related to the flow for different slurry flow measurement sites, and a selection method is given. Aiming at the problem that the zero point is too large in the water calibrating, the problem is solved by de-differentiating the data. The signal processing method is realized in the DSP-based hardware system of electromagnetic flowmeter. The experimental results of water flow calibration and slurry flow signal processing show that the accuracy meets level 0.5 when measuring water flow. The fluctuation of calculation results is less than 5% and the response time is less than 5 seconds when measuring slurry flow. The effectiveness and general applicability of the method are verified.