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The temperature field distribution and thermal history of Fe-9Cr2WVTa reduced activation steel prepared by laser melting deposition (LMD) have been calculated with Gaussian and Ring laser beams, and the nucleation and growth behaviors of M23C6precipitates in the 1st, 7th and 19th layers have been calculated using the modified classical nucleation theory and Svoboda Fischer Fratzl Kozeschnik model. The energy distribution shows W-shape with Ring laser beam while it shows V-shape with Gaussian laser beam, which results in the more uniform M23C6size in the same layer with Ring laser beam. Precipitates in the bottom (i.e., the 1st layer) have the minimum size and the size increases with the layer number with Gaussian and Ring laser beams. The temperature history, the instantaneous nucleation rate and the size evolution of M23C6have been systematically discussed. The results indicate that the nucleation, growth and re-dissolution of precipitates in reduced activation steel depend on the amount of energy absorbed in the thermal cycle during LMD. The continuous accumulation of energy during the thermal cycle leads to larger M23C6at the top area. The unsteady state precipitation dynamics of M23C6carbides during thermal cycling are consistent with the simulation results.