In order to investigate the effect of water-cement ratios on the microstructure and micromechanical properties of oil well cement slurry, mercury intrusion, thermogravimetric and X-ray diffraction techniques were used to characterize the internal pore structure, hydration process and phase content of oil well cement slurry. The statistical nanoindentation technique was utilized to probe the micromechanical properties of oil well cement slurry, in the meanwhile, both least squares estimation and maximum likelihood estimation methods were adopted for the deconvolution calculations, through which, elastic modulus, hardness, and volume fraction of solid phase in oil well cement slurry were gained. The BSI model is chosen to evaluate and select the most appropriate bin size because the bin size plays a significant role on the accuracy of the least squares estimation algorithm. The results of this study indicate that the water-cement ratio affects the porosity, pore size distribution, and pore properties of oil well cement slurry. A higher water-cement ratio favors the formation of hydration products. In addition, the elastic modulus of the hydration product decreases significantly with the increasement of water-cement ratio. The hydrated calcium silicate gel has a higher density under the low water-cement ratio condition and vice versa. Although the two deconvolution algorithms are based on distinct criteria, both of them are suitable for deconvolution calculations of cementitious materials.