In the process of microhole EDM with difficult-to-cut materials, the electrode wires of EDM machine usually position in guide sleeve, which is introduced from the end of motorized spindle of machine tool. Based on this problem, a through-hole ultrasonic system was designed. The designed exponential transition stepped horn combines the advantages of two monotype horns, and it is convenient for the placement of electrode wires. Combined with the purchased ultrasonic generator and transducer, the resonant frequency of the horn was designed as 43 kHz, the large end-face was designed as 20 mm according to the transducer, and the small end-face was designed as 12 mm. After calculating the other parameters, ANSYS was used for finite element analysis. Then the parameters were optimized. Finally, the impedance test of the machined ultrasonic system was carried out, and then it was installed on the EDM machine tool to process microholes with different materials and the inlet morphology was observed. The calculation and test results show that: the optimized horn has smaller frequency error and higher stability;the holes with ultrasonic processing are better than ordinary EDM processing in terms of machining efficiency and surface morphology. It is verified that the exponential transition stepped horn meets the original design intention.