Abstract: Microstrip line interdigital filter is selected for use as 10～12 GHz filter in multi channel receivers. A method of designing microstrip line interdigital filter is proposed. According to the symmetry of current in microstrip line, microstrip line interdigital filter is divided into parallel coupling pairs independent of each other. Given the impedance of a parallel coupling pair, parameters of each parallel coupling pair can be obtained simply by the process of synthesis proposed in the article, and so parameters of microstrip line interdigital filter given. This method is simple in designing and easily available for hardware realization. The ADS software is used to simulate and optimize the computed data. Finally, the tested curve is given. The method proposed is recommendable for engineering applications.