With the demand of improving the second interfacial bonding strength and environmental protection, MTC cementing technology has obvious technical and economic advantages. However, due to containing lots of low-density materials and slurry treating agents, which make low strength, the application of low-density slag MTC cementing slurry system is restricted in the cementing of deep wells and low-pressure leakiness formations. According to the theory of particle distribution and based on particle size analysis of floating bead, micro silicon and slag, solid-phase proportioning design of low-density (1.35~1.55 g/cm3) slag MTC was carried out in this paper. By adjusting the adding amount of micro silicon and activators, the slag MTC cementing fluid system with the 24-hour compressive strength greater than 21.6MPa which has the density range of 1.35 to 1.55g/cm3 was get. This system has small initial consistency, short thickening transition time, low water loss and good rheological property. And at last, the mechanism of improving the strength of low-density slag MTC cementing fluid system was analyzed.