The embodiment of the invention provides a method for measuring the slag viscosity. The method includes the following steps that the intensity of light reflected by blast-furnace slag at the same position at different angles is obtained; the temperature of the blast-furnace slag is obtained; according to the intensity of light reflected by the blast-furnace slag at different angles, the refractiveindex of the blast-furnace slag at different angles is obtained, and a main component of the blast-furnace slag is determined through a pre-established relationship template library of the main component of the slag and the refractive index; according to the main component and temperature of the blast-furnace slag, the viscosity of the blast-furnace slag is determined. Based on the principle thatthe directional refractive indexes of different objects are different, the directional refractive index of the blast-furnace slag is detected through a photosensitive element, and the viscosity of the blast-furnace slag can be calculated in real time in combination with the temperature of the blast-furnace slag.