Abstract: Via-holes are usually required to ground for microstrip interdigital filter.The influence of the grounding via-hole is considered when modeling the overall system in traditional design methods, which will lead to the multi-factor and multi-level problem since the number, size and position of the grounding via-holes are uncertain.The problems can be avoided by proposing grounding via-hole effect in microstrip interdigital filter design.Taking Ka-band filter as an example, the traditional odd-even mode impedance method, coupling coefficient and external quality factor method, and the modification method of considering the grounding via-holes in the design process are used to design the filter respectively.The initial design results show that the center frequency of the traditional method is shifted to the left, and other parameters are much different from the design specification.The center frequency, passband range and stop rejection of the modification method meet the requirements of the specification, and the maximum insertion loss is 3.08 dB, the minimum return loss is 10.08 dB, which is helpful to reduce the subsequent iterations.