Abstract: Abundance sensitivity is an important technical index of magnetic mass spectrometer, which describes the extent to which the tailing of intense ion peaks at a mass number in its neighborhood affects the weak ion peaks during isotope ratio measurement. It is particularly important in the measurement of isotope ratios containing low abundance nuclides, such as ²³⁴U/²³⁸U, ²³⁶U/²³⁸U, and ²³⁰Th/²³²Th. However, the abundance sensitivity of magnetic mass spectrometer is only of the order of 10⁻⁶ without taking additional technical measures. To further improve the abundance sensitivity, the techniques of tandem mass spectrometer, such as magnetic-electric, magnetic-magnetic and magnetic-electric-quadrupole tandem mass spectrometer, and retarding filter can be adopted. In fact, while various tandem mass spectrometer techniques can effectively improve the abundance sensitivity of an instrument, they also significantly increase the size, complexity, and cost of instrument, and therefore are not widely used. In contrast, due to its simple structure, small size, and excellent performance, the retarding filter has become a key component for commercial magnetic mass spectrometers to improve abundance sensitivity. Based on the basic principles of ion optics, a retarding filter consisting of five electrodes with a compact structure of only (40×50×50) mm³ was developed by utilizing the ion optics simulation software SIMION8.1. In addition to the function of energy retarding, the retarding filter also has the function of direction retarding, which greatly enhances the filtering capability to scatter ions. To test the ion transmission efficiency and the abundance sensitivity under different voltages, the retarding filter was installed on a domestic double-focusing thermal ionization mass spectrometer (TIMS). The instrument mainly consists of a thermal ionization source, a double focusing mass analyzer consisting of an electrostatic analyzer and a magnetic mass analyzer, a zoom lens, and a multi-collector detector, which can be configured with up to 16 faraday cups, 4 electron multipliers, and 2 retarding filters. The results showed that the abundance sensitivity of the mass spectrometer is successfully improved from <2×10⁻⁶ to <5×10⁻⁹ without either main ion beam transmission efficiency reduction or the peak shape deformation. The best abundance sensitivity obtained is 2×10⁻⁹, which is highly competitive to commercial thermal ionization mass spectrometers in the market today. The as-designed retarding filter provides important technical support for the development of domestic high-performance magnetic mass spectrometers.