In view of the complicated issue that how to improve the cementing quality of the second interface in cementing engineering, experimental and theoretical research on slag MTC slurry improving the quality of the second interface was carried out in the paper. Experimental study shows that slag MTC and mud cake can achieve overall cementing; the volume shrinkage ratio of slag MTC is far less than the cement slurry system; and the strength of slag MTC cannot be affected by the presence of mud cake and is greater than low-density cement slurry. Theoretical analysis shows that the alkali activator and Ca(OH)2 produced by hydration can react with SiO2 which exists in mud cake and dead mud to get C-S-H gel with a low ratio of calcium to silicon, and improving the quality of the second interface. The study on the chemical shrinkage ratio of the products of slag MTC hydration was made by using the method of chemical analysis, and low-shrinkage mechanism of MTC was revealed. The results show that mud cake solidification, dead mud solidification and low-shrinkage ratio are fundamental to improve the cementing quality of slag MTC and formation.