Molten nitrate salts are widely used as heat transfer and energy storage medium in Concentrated Solar Power (CSP) systems. Solar Salt (60 wt% NaNO3-40 wt% KNO3) is the commercial binary molten nitrate salt, which is the preferred energy storage material with high density, high specific heat, low melting point, high thermal stability, and low vapor pressure. This paper explored the effects of impurity Cl- on the thermophysical properties of Solar Salt, including liquidus temperature, density, viscosity, and thermal stability. The results showed that Cl- can significantly reduce the liquidus temperature, and when Cl- was less than 0.5 wt%, the liquidus temperature of molten salt system decreased within 1 degrees C. On the other hand, Cl- had little effect on the density, viscosity and thermal stability of the mixed molten salt system at 400 degrees C, but at high temperature Cl- will promote the volatilization of components. By analyzing the thermostatic stability at 565 degrees C, it was found that the total mass loss changes less than 0.3% when Cl- was less than 0.01 wt%. After comprehensive analysis, the conclusion is that the upper limit of Cl- is preferably less than 0.1 wt% for keeping good thermal performances of Solar Salt.