The influence of Nano- CaCO3 on the mechanical property of PALC was studied in the paper. And the mechanism of Nano- CaCO3 modified cement-based materials was also discussed by means of X-ray diffraction and SEM. The results shows that the PALC with 2% of added Nano-CaCO3 by weight of cement has an optimum splitting strength, in which the increase of splitting strength are about 15.2%,20.4% and 19.7% at 3days, 7days and 28 days. The reasons for causing the above results are because Nano-CaCO3's filling effect of micro-particle, surface effect of nano-particles as well as micro-nucleus effect.