Heat of hydration test, non-evaporating water content test, X-ray diffraction analysis, scanning electron microscope analysis, mercury intrusion test and compressive strength tests were used to investigate the effect of the Co-Fe-Mg-Al hydrotalcite carbon nanotubes (CoFeMgAl-LDHs/CNTs) composite material on the hydration process, pore structure and strength of hardened cement paste. The results show that CoFeMgAl-LDHs/CNTs composite material significantly improves the hydration degree of cement within 3 d through nucleation, and thereby significantly enhances the compressive strength of the cement paste in the age of 3 d, the effect of improving the strength gradually reduces after 3 d and become unobvious after 7 d. Compare with blank cement paste, the content of gel pore and capillary pore in hardened cement paste increase obviously, while the macro pore decreases. At the same time, bridging effect of composite material optimizes microstructure of cement paste, and increase the stress bearing capacity of weak parts of cement matrix. Therefore, at the same age, the strength of hardened cement paste is increased. Because the change of composite content has no obvious effect on the change of pore size distribution, the change of composite content has little effect on the strength improvement of hardened cement paste at the same age.