Abstract: Flow conditioners are widely used to improve flow uniformities, ensuring flow meter measurement accuracy. In this paper, a series of 3-D computational fluid dynamics investigations using SST k–ω turbulence model were carried out to reveal the flow characteristics of the main feed water system of a nuclear power plant with AMCA and three types of perforated flow conditioner. Firstly, results showed that the configuration affected the velocity profile and pressure of downstream, flow uniformity, flow meter discharge coefficients. A perforated flow conditioner with extended length and increased porosity was selected for its rectification performance. Secondly, results illustrated the installation position affects the uniformity as well as pressure and velocity distribution. Thirdly, the extra pressure loss caused by the flow conditioner was analyzed, and the relation between the flow conditioners and the main valve openings was discussed at full load.