Abstract: There are many initial design methods for microstrip interdigital filter with tapped-line but the design methods are not complate, concise and accurate enough, so a set of initial design flow with completeness and conciseness is given. The design steps are as follows: index "standardization", determine the low-pass prototype filter, determine the order of the filter, determine the normalized conductance value, design tapped line length and width, design resonator width, design non-input/output resonator length, design adjacent resonator spacing, design input/output resonator length and tapped line position, integral modeling and simulation of interdigital filter. Finally, taking the initial design of Ka-band filter as an example, the initial design results show that the center frequency is about 30.19 GHz, the passband range is slightly larger than 29.40-31.00 GHz, the maximum insertion loss is 4.21 dB, the minimum return loss is 9.35 dB, the stopband rejection of 27.00 GHz and 33.40 GHz is more than 30.00 dB, which basically meet the design indexes, only a little optimization is needed to fully meet the design indexes, and the feasibility and high accuracy of the design flow are verified.