Abstract: In the process of nuclear installation decommissioning project, the specific composition of a batch of unknown radioactive yellow powder which stored in a former waste house and had no demark, was verified by γ spectrum radionuclide analysis, spectrophotometry, high temperature decomposition verification test and mass spectrum analysis. This procedure provide reference frame for how to treat the unknown radioactive yellow powder in further process. And the conclusion is that the main ingredient of the sample is ammonium biuranate, and its ²³⁵U abundance is lower than 0.72%, it should be depleted uranium.