Abstract: The fundmental of designing hairpin microstrip filter and the application of ADS in the design of hairpin microstrip filter with frequency in the L-band is introduced. The approach of obtaining the relationship between microstrip resonator's spacing and microstrip resonator's coupling coefficient is given. The results of model simulation and EM simulation are shown. At last,the design example and measured result for a hairpin bandpass filter of fifth order with frequency in the L-band are given.