Molten salt pumps are key equipment used in solar high-temperature heat transfer and heat storage systems. As it is difficult to use the high-temperature molten salts for experimental tests, and significant changes in the molten salt viscosity affect the pump performance, a modeling scheme is proposed and verified in this study. An unsteady flow field simulation inside the pump is conducted for molten salt pump conveying mediums with different viscosities. To study the unsteady characteristics of the flow field inside the pump, the angle alpha between the blade and the volute tongue is defined. It is found that the shape of the pressure curve at the volute tongue is related to the medium viscosity as alpha changes. The position of the blade affects the velocity distribution inside the volute significantly. Moreover, the shapes of the performance curves H-Q, N-Q, eta-Q, are greatly affected by the viscosity of the molten salt. The mechanism of flow loss with different viscosities mediums inside the pump is revealed. The research results provide an important reference for the further studies of molten salt pump. (C) 2019 Elsevier Ltd. All rights reserved.