Microstrip interdigital filter requires use of grounding via-holes, but the grounding via-holes will influence performance of interdigital filter. The traditional design method does not consider grounding via-holes effect in the initial design process, which will lead to some uncertainties of the grounding via-holes, such as the number, the size and the position. This will make the subsequent optimization face multi-factor and multi-level problems. In addition, when the substrate thickness and the center frequency increase to a certain extent, the external quality factor obtained by using the traditional 50 Ω tapped-line cannot reach the theoretical value. Therefore, the traditional design method is modified, and the grounding via-holes effect is considered in the initial design process by using 3D electromagnetic field simulation software. When the traditional 50 Ω tapped-line do not meet the design requirements, it is improved to a gradual tappedline with the combination of a 50 Ω microstrip line and a narrower microstrip line. Taking a Ka-band filter as an example, 100 μm and 300 μm thick high resistance silicon are used as substrate respectively. The simulation results indicate that the initial design for filter with the modified method is closer to the specifications, which can reduce subsequent iterations. Besides, the insertion loss of filter with gradual tapped-line is 1.91 dB, and return loss is 18.06 dB. What’s more, the stop rejection at 27.00 GHz and 33.40 GHz are 46.91 dB and 59.58 dB, respectively.