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During cementing operations, unavoidable contact between water-based mud (WBM) and oil well cement (OWC) often degrades OWC performance. This is particularly problematic in natural gas or offshore wells, where zonal isolation failure can result in gas or liquid leakage, posing significant environmental and safety risks. To alleviate this problem, a masking agent was incorporated into the OWC to suppress contamination. The masking agent functions by shielding the active sites on cement particles and chelating metal ions, thereby inhibiting the cross-linking between WBM and OWC. Experimental results demonstrated that the addition of 0.5% of the masking agent increased the thickening time of the contaminated OWC system by 276%, while also doubling both its fluidity and compressive strength. To gain a more comprehensive understanding of the underlying mechanism, quantum chemistry and molecular dynamics simulations were employed. The results indicate that the masking agent exhibits superior adsorption properties, allowing it to compete effectively with the polymers for reaction sites. Overall, this study emphasizes the critical role of the masking agent in mitigating WBM contamination and proposes a viable strategy to improve the safety and quality of cementing operations. Furthermore, this approach can reduce or eliminate the need for spacer fluids, thereby addressing environmental challenges associated with the disposal of wellbore waste fluids.