Abstract: To explore the formation mechanism of uranium, electrospray ionization tandem mass spectrometry was used to simulate and analyze the chemical bond breaking of uranium-containing molecules. The results show that U⁺ is generated in collision induced dissociation mode after uranium-containing molecules were ionized in mass spectrometry. By further study it is found that there are UO⁺₂ (m/z 270), UO₂OH⁺(m/z 287), UO₂H₂OOH⁺(m/z 305), UO₂NO⁺₃(m/z 332), UO₂(H₂O)₃NO⁺₃(m/z 386) and bis-uranyl ion (UO₂NO₃H₂O)₂NO₃⁺(m/z 762) in the solution, all of which could form U⁺ under the collision of N₂ with kinetic energy. The formation mechanism of natural native uranium deducted from simulation experiment is the following: Free uranium cations are generated from the breaking of uranium-containing molecules by collision of neutral particles which are the decay products of radioactive substance, and further, uranium cations form natural native uranium and is preserved in closed or strong reduction geological conditions in a long geological period.