The present study delivered the measurements of viscosities in SiO2–K2O–CaO system which is important for many applications including biomass slags. The rotational spindle technique was used in the measurements in the composition range of 49.8–78.2 mol% SiO2 and 7.7–29.5 mol% K2O and temperature range of 997–1601 °C. Molybdenum crucibles and spindles were employed in all measurements in ultrahigh purity argon. Analysis of the quenched sample by Electron Probe X-ray Microanalysis (EPMA) after the viscosity measurement enables the composition and microstructure of the slag to be directly linked with the viscosity. The increase of SiO2 concentration has been proved to significantly increase both viscosity and activation energy. The replacement of CaO by K2O was found to decrease viscosity and activation energy of the SiO2–K2O–CaO slags. The viscosities of the industrial biomass slag are compared with the synthetic slags. Predictions of the viscosity from several models are also compared with the experimental data from the present study and literatures.