In this study, a simple and yet novel methodology based on the use of modified inclined plane (M-IP) has been established and validated to determine the high-temperature slag viscosity. Seven synthetic standard ash samples have been tested on the inclined corundum plane, under the conditions of reducing environment of 1% CO in nitrogen, 1100 degrees C-1400 degrees C, 25 degrees-90 degrees and different duration time of 10-40 min. The slag mass was varied from 100 mg to 400 mg. A multiple linear regression fitting was conducted to establish an empirical equation to predict slag viscosity (eta) based on the slag travel length per unit mass (L') and the inclination angle of the plane (cos beta). As has been fully validated, the new method is simple and requires a maximum of 200 mg ash for the slagging test. By measuring only two temperature points, an Arrhenius equation can be established to predict the travel length at any of the intermediate temperatures. Both the inclination angle and the reciprocal of slag viscosity affect the slag travel length in an intertwined manner, by following a linear relationship between the algorithm of slag travel length per unit mass, cos beta and the logarithm of slag viscosity, as per the equation of ln mu = 3.282281cos beta-1.882827lnL' + 7.397108. The large correlation coefficient value and the normal quantile plot supported the high reliability of this equation. However, this empirical equation is limited to an upper temperature of 1400 degrees C and a maximum viscosity of 17.9 Pa.s. Any larger viscosity would only be measured qualitatively, whilst it is sufficient for a comparison with the critical viscosity 25 Pa.s from the perspective of a quick screening of solid fuel for entrained-flow gasifier or cyclone furnace.