Abstract: The ratio of ²³⁶U and ²³⁸U atom number is different depending on the type of reactor and the status of irradiation of nuclear materials, which is usual 10⁷-10¹¹ times than that in nature. The irradiation activity near sampling spots, the source of irradiation contamination, and the burnup of spent nuclear fuel can be explored by measuring ratio of ²³⁶U and ²³⁸U atom number in environmental samples. The method of measuring ratio of ²³⁶U and ²³⁸U atom number in simulated samples by MC-ICP-MS and the working schedule for estimating burnup of spent nuclear fuel have been established. Compared with the other methods, the relative deviation of burnup is about 10%.