In order to solve the overheating and tube failure problems caused by deposits accumulated on the orifices in waterwalls of ultra-supercritical boilers，numerical models of a sample orifice are established based on the FLUENT platform after the analysis on the deposition mechanism. A detailed description of the flow field near the orifice is obtained with the help of the vector plots and the pressure contours. Trajectories and concentration distributions of particles injected from different sites are calculated and analyzed. The results indicate that precipitation and deposition of magnetite caused by the sharp pressure drop near the orifice surface is the main reason for the deposition problem.