A multidimensional numerical model was developed to investigate the temperature field, fluid field of liquid phase in the molten pool, and microstructure evolution in the plasma deposition manufacturing (PDM) process. A level--set approach was used to track the evolution of free surface of the molten pool, and an enthalpy--porosity model was introduced to deal with the transformation of solid and liquid phases. To understand the physical mechanism of thermal impact on the microstructure of the deposited layer, a Monte Carlo method combined with thermal--fluid analysis was applied to track the grain growth process in the PDM process. A numerical experiment of nickel--based alloy thin wall parts by PDM was implemented. The numerical results show that the microstructure of the deposited layer mainly depends on frequency  and amplitude of thermal impact, which is also influenced by variable processing parameters such as plasma power, scanning speed, and powder feed rate. Therefore, under full melting of fed powder, an increase of scanning speed could make the grain size of final microstructure finer to some extent.