Abstract: The concentration of ²³⁵U is the key information for nuclear safeguards and nuclear material accountancy, and the enrichment of ²³⁵U is usually measured and analysed based on the principle of decay equilibrium using the 185 keV energy peak of ²³⁵U and the 1001 keV energy peak of ^234mPa of the daughter of ²³⁸U. However, in the production of enriched uranium, the equilibrium between the daughter and the parent body is broken after separation by gas diffusion method or centrifugation method, and the continuous generation and decay of the daughter in the newly separated samples make the measured content inaccurate, and the direct use of the ^234mPa content in the daughter of ²³⁸U leads to an error in the results obtained, which is not in line with the actual enrichment degree. In order to be able to measure uranium samples in nonequilibrium, this paper proposes a time-correction method based on the ^234mPa of the ²³⁸U substrate, which is based on the relationship between the sample separation time and the decay change to obtains the enrichment degree of uranium in nonequilibrium after the time correction from the relationship between the sample separation time and the decay change. The method was used to analyze and validate a 1.8% enriched UF₆ gas with a separation time of about 40 days and compared the results with those obtained by PC/FRAM software. The results reduced the deviation from the nominal enrichment value from an average of 47.5% to 4.1%, achieving a more accurate correction of the calculated enrichment based on the separation time.