The invention discloses ultra-low viscosity cement paste. Raw materials of per cubic meter of cement paste comprise the following components: 900-1100kg of silicate cement, 90-200kg of granulated blast furnace slag powder, 90-200kg of limestone powder, 75-180kg of fly ash microsphere, 10-50kg of silica fume, 20-50kg of high performance water reducer, 5-20kg of high performance viscosity reducer, and 300-600kg of mixing water. A preparation method of the ultra-low viscosity cement paste comprises the steps of weighing all raw materials according to a raw material proportion for later use; putting the mixing water, high performance water reducer and high performance viscosity reducer into an agitation pot of a concrete mixer; then adding the silicate cement, the granulated blast furnace slagpowder, limestone powder and silica fume into the agitation pot; and starting the concrete mixer, agitating for 3 minutes so as to prepare the ultra-low viscosity cement paste. Compared with the prior art, the cement paste has advantages of ultra-low viscosity and so on.