Abstract: Based on the laser resonance ionization mass spectrometer (LRIMS) developed in the laboratory, a preliminary method was established to determine plutonium isotope ratios. Through scanning the wavelength of ionizing laser, a three-color-three-photon resonance ionization scheme for plutonium with greater ionization section was obtained. A uranium plutonium mixture was prepared to be used as a simulant sample, and the plutonium isotope ratio in the sample was determined. Results show that the relative standard deviation of ²⁴⁰Pu/²³⁹Pu isotope ratio is 1.1%, and the element selectivity of plutonium to uranium is better than 5×10⁶. This work indicates that LRIMS can effectively avoid the isobaric interference and strong-peak-tail interference from other elements.