Molten salt centrifugal pump is used to deliver high temperature molten salt, whose performance is highly affected by the fluid viscosity. In order to investigate the external performance and internal flow of molten salt pump for different viscous fluids, especially reveal the effect of viscosity on unsteady per-formance, the unsteady flow in a molten salt pump was researched when the pump handles the fluids with different viscosities by numerical simulation to quantitatively clarify the effects in this article.The external performance estimated using the unsteady flow model was compared with experimental results. It shows that the numerical simulation results agree well with the experimental results. The shaft power increases with the increase of density and viscosity. Compared with delivering water, the head and efficiency show less change for delivering low viscosity fluids, while for delivering high viscosity fluids, they all decrease significantly. Flow steadiness in the molten salt pump is mainly affected by the viscosity and the interference effect of blade. An increasing viscosity of fluid reduces the fluctuation in flow variables.The results of the study have reference significance on the development of high-performance molten salt pump and stable operation.