Microstrip cross-finger filters, as a common two-dimensional filter, have a wide range of engineering applications, but due to their large size, thus limiting the expansion of application scenarios. In this paper, a new microstrip cross-fingerfilter is designed based on the theory of step impedance resonator (SIR) , which completes the optimization ofthe size of the conventional microstrip cross-finger filter. The filter uses a three-section SIR structure, which reduces the lengthby23.2% compared to the resonatorofa conventionally structured filter. The performanceof SIR filterand traditionalfilter under the same design index is simulated using CST, and the results show thatboth of them can meet the design index requirements, and the performance of SIR filter is better in the passband, which indicates thatthe use ofthismethod to realizethe miniaturization has strongpracticalsignificance. In orderto verifythe usability ofthe filter, physicalprocessing and testing werecarried outwith aboard sizeof24mm×18mm×0.508 mm , and resonant structure size of 10.9 mm × 13.6 mm , the measured results and the simulation results show that the difference betweenthemeasured and simulated resultsislessthan1.5 dB atthepassband. Thefinalspecificationsofthis filterare center frequency 2.5 GHz, bandwidth 150 MHz, in-band loss ≤ 4 dB, and stopband rejection ≥ 24 dB (frequency≤2.2 GHz or≥2.72GHz) .