Abstract: The age of radioactive materials is an important traceable fingerprint feature. The study of uranium particle age measurement is of great significance for the application of nuclear forensics technology. The age analysis of uranium particles is one of the most advanced research contents in environmental sample analysis technology in the field of nuclear safeguards. The age of uranium particles refers to the last separation and purification time of uranium. By collecting and measuring the age of wiping samples from the internal or surrounding environment of nuclear facilities, the key information of the occurrence time of corresponding nuclear activities can be obtained, and then whether there is a default production activities can be determined. The principle of particles age measurement is the same as bulk samples, its age is calculated by measuring the atomic ratio of uranium and its decay daughters. The age of uranium particles is usually analyzed by measuring the ratio of ²³⁴U and ²³⁰Th. In this work, based on the preparation of monodisperse micron-sized mixed particles of uranium and thorium oxide, the thorium uranium ratios in self-made monodisperse mixed particles of uranium and thorium oxide and the uranium and thorium mixed solution were measured by secondary ion mass spectrometer (SIMS) and inductively coupled plasma mass spectrometry (ICP-MS). The relative sensitivity factor (RSF_Th/U) of uranium thorium ratio in a single micron particle was calculated. Combined with scanning electron microscope (SEM) and micromanipulator, the influence of sample preparation process on the measurement results is successfully reduced, the optimal measurement conditions are determined, and the SIMS measurement method of micron mixed particles of uranium and thorium oxide is explored. The results of mixed particles of uranium and thorium oxide measured by SIMS show that for mixed particles with a diameter of 2-3 μm, the relative standard deviation (RSD) of the ratio of ²³²Th/²³⁸U between different particles is less than 3%(n=12), and the average RSF_Th/U is 1.259±0.032. RSF_Th/U was verified by measuring CRM970 uranium powder standard material with a known age. The results show that for CRM970 uranium powder samples with a diameter of 5-10 μm, the age measurement results are accurate, and the RSD is 3%(n=16). The results show that the SIMS measurement method of mixed particles established in this work is less affected by interference signals and the measurement results are stable. It can be used to measure the age of micron uranium particles.