The invention provides a high-aluminum slag viscosity adjusting method under the blast furnace low agglomerate ratio condition, and relates to the technical field of blast furnace ironmaking. When the in-furnace agglomerate ratio is not lower than 75%, the in-furnace agglomerate ratio is periodically lowered by 0.9-1.1%, sampling is performed, the slag viscosity value of slags under the temperature of 1,500 DEG C is detected, and the slag viscosity is controlled to be smaller than 0.50 Pa.s; when the in-furnace agglomerate ratio is lower than 75% and higher than 70%, dolomite is cooperatively added into a feeding system for adjustment, and the addition amount of dolomite is 0-15% of the current blast furnace in-furnace mineral batch weight; when the in-furnace agglomerate ratio is not higher than 70%, according to changes of the high-aluminum slag viscosity changes under the temperature of 1,500 DEG C, dolomite is cooperatively added into the feeding system. The slag ingredient is effectively adjusted, the slag viscosity is rapidly lowered, and the slag flowability is improved. By directly adding dolomite into the blast furnace, the in-furnace raw material structure is adjusted, the viscosity of high-aluminum slags under the low agglomerate ratio smelting state can be effectively adjusted within the reasonable range, and it is ensured that the high-aluminum slag manufacturing system under the low agglomerate ratio raw material condition is stable.